My fix was listed above. restarting the routers and defining the static IP

By heart it took about 10-12 days. "maar pin me er niet op vast"

I am using the developer branch because I switched to the MQTT Client. Becuase I still have longer range connection issues I would like to set the datarate to 250 KBs. I change the setting in MyConfig.h to :

#define MY_RF24_DATARATE RF24_250KBPS

But if I look in my debug information I see:
CONFIG 0x0E
DYNPD/FEATURE 0x3F 06
Data Rate 1MBPS
Model nRF24L01+
CRC Length 16 bits

Do I need to define the datarate somewhere else ?

@sundberg84 : I have also received the rev 7 from DirtyPCB and learned how to solder. Very nice and flexible design !

Perfect for a Newbie :). Thnaks a lot for sharing

My set-up is stable now for a couple of days, even with reboots of MQTT server and my network switch

The set-up in Openhab was working because I was able to connect to another MQTT server on my Synology. I just changed the code a little by also defining the DNS server and subnet mask

#define TCP_PORT 1883 // Set your MQTT Broker Listening port.
IPAddress TCP_IP ( 192, 168, 0, 9 ); // Configure your static ip-address here
byte gateway[] = { 182, 168, 0, 1 };
byte subnet[] = { 255, 255, 255, 0 };
byte TCP_MAC[] = { 0x02, 0xDE, 0xAD, 0x00, 0x00, 0x42 }; // Mac-address - You should change this! see note *2 above!

and

void setup()
{
Ethernet.begin(TCP_MAC, TCP_IP, gateway, subnet);

and it seems and I can connect now during the last two restarts (but I also powered-off my netwerk switch in between). Let's see how it will last.....

I gradually started to build my first system. After getting experience with the serial gateway, I shifted to the th ethernet Gateway and now want to setup the MQTT gateway. Everything is functioning,** I can see the MQTT updates in MQTTlens (temperature readout)** but by no means I can connect my Openhab system to the MQTT broker on the Arduino UNO. I get a timeout connection error all the time:

21:14:10.253 ERROR o.o.i.t.mqtt.MqttService[:114] - Error starting broker connection
org.eclipse.paho.client.mqttv3.MqttException: Unable to connect to server
at org.eclipse.paho.client.mqttv3.internal.TCPNetworkModule.start(TCPNetworkModule.java:75)
at org.eclipse.paho.client.mqttv3.internal.ClientComms$ConnectBG.run(ClientComms.java:538)
at java.lang.Thread.run(Unknown Source)
Caused by: java.net.ConnectException: Connection timed out: connect
at java.net.DualStackPlainSocketImpl.waitForConnect(Native Method)
at java.net.DualStackPlainSocketImpl.socketConnect(Unknown Source)
at java.net.AbstractPlainSocketImpl.doConnect(Unknown Source)
at java.net.AbstractPlainSocketImpl.connectToAddress(Unknown Source)
at java.net.AbstractPlainSocketImpl.connect(Unknown Source)
at java.net.PlainSocketImpl.connect(Unknown Source)
at java.net.SocksSocketImpl.connect(Unknown Source)
at java.net.Socket.connect(Unknown Source)
at org.eclipse.paho.client.mqttv3.internal.TCPNetworkModule.start(TCPNetworkModule.java:66)
at org.eclipse.paho.client.mqttv3.internal.ClientComms$ConnectBG.run(ClientComms.java:538)
at java.lang.Thread.run(Unknown Source)

I have googled for any solution and I checked double-checked all my settings but I am out of ideas, specially because I can connect/receive messages from the MQTT broker to my android phone and my desktop. Anyone any suggestions where to look ?

After a steep learning curve I found out that one of my UNO clones was not able to receive messages in any way. I have now made a sensor based on a pro mini which works fine with a serial gateway. The only problems I still encounter is that the node-ID is not assigned automatically but I solved it by defining it in the sketch. @sundberg84 : I saw a different post of you for the Easy/Newbie PCB for MySensors. It looks very tempting to start using them !

@sundberg84 : in the same room. One on the serial monitor, one in an external power supply

Thanks a lot for your answers. I have installed the caps and both the node and gateway are initiated every time flawless but they refuse to communicate to each other. The modules are recognized as being '+' versions using the GetStated sketch from the RF24library. Tried to setup both uno's in different ways (pingpong; serial gateway and sensor) but none of the combinations shows any way of communication between the two boards. I have been struggling for three days now; I am out of ideas and I am afraid it's a problem with the nRF modules (see this link also ) . I will try to order some new ones.....

Hi Martien, It seems that I have the same problem. I bought my NRF24L01+ through the aliexpress link on this site. The initiate like they should but do not communicate at all. Where did you order your new ones ?

I just started a week ago building my own sensors. I can upload sketches to my two UNO clones , program some buttons, leds, etc. but I can't get my sensors to communicate to each other using PingPongSensor. Probably to do with the power supply or the connection of the capacitors to the NF module, so I will continue to struggle. Meanwhile I have two questions:

1. what is the best capacitor size on the NRF+, I sometime see 4,7 uF sometimes 47 uF and even 100 uF
2. I have two complier warnings (see below) can they cause an issue ?

**Warning: platform.txt from core 'MySensors AVR based boards' contains deprecated recipe.ar.pattern="{compiler.path}{compiler.ar.cmd}" {compiler.ar.flags} {compiler.ar.extra_flags} "{build.path}/{archive_file}" "{object_file}", automatically converted to recipe.ar.pattern="{compiler.path}{compiler.ar.cmd}" {compiler.ar.flags} {compiler.ar.extra_flags} "{archive_file_path}" "{object_file}". Consider upgrading this core.
WARNING: Category '' in library UIPEthernet is not valid. Setting to 'Uncategorized'
I love this initiative. I do have an OpenHab system with some z-wave sensors and HUE lamps, but it’s way too expensive to buy it all, so this is a very nice opportunity to expand my home !
**