Issue solved Arjan,
apparently the RAK gateway is not sending the uplinks fast enough when reveiving ‘the join permission’ from the TTN network server.
I tried another gateway (a lorank8) and immediately after the ‘join accept’, the uplinks came in right away after 2 ms.
Reason why the RAK is reacting so slow, has not been traced. Conclusion, it is not the timing of LMIC on the node, but a packet forwarder timing issue on the gateway.
Referring to the TTIG problems : TTIG does not support downlinks when not frequently receiving uplinks?