Well, maybe there is no usefull info in the troubleshooting files, and maybe the low RSRP is the fundamental problem, but the RUT240 still behaves very strange. As I wrote in the first post, then it breaks down when I load it. At first I thought it was then trying to re-register on the network and only managed to do so after several hours. But it seems that it is still registered. This is what I experienced when RUT240 is in this strange mode:
WEB UI only partly functional: Login works, Menu works, but no data is shown in any fields.
SMS gateway doesn't work (no reply or action follows sending an SMS to RUT240).
OPENVPN Client seems to work.
Routing functions seems to work.
I can SSH to RUT240 (through the VPN tunnel).
I can reboot RUT240 via SSH.
After the reboot evrything works again.
Unfortunately it will take some months before I will be physically at the RUT240 location.