Does the issue occur on all of the devices?
The APN seems correct, signal metrics are very good too. However disconnects seem to happen due to failover.
Is the failover configuration necessary for your application? Maybe you could disable it and configure automatic modem reboot (Services -> Auto reboot -> Ping/Wget reboot) to monitor connection state instead as both devices are not connected to a secondary WAN at the moment?
If failover is mandatory, could you modify the configuration of Network -> Failover -> mob1s1a1 settings in the following way:
- Add 188.8.131.52 or your provider's DNS IP to Track IP options list. You can look up your ISP DNS servers via CLI/SSH command: cat /tmp/resolv.conf.d/resolv.conf.auto;
- Increase Down field value to 3;
Could you try one of the devices with another provider's SIM card?
If the above does not help, we could try updating one of the routers modem firmware.