FWG: secondary ISP for failback goes down every 2-3 hours

Hi, 

due to some recent ISP downtime, I decided to take a 4g Router to create a back-up connection. The current config is:

FWG

LAN4 to primary ISP with optical fiber and PPPoE connection with IPV6 enabled

LAN2 to Asus AX86U in bridge mode, which provides WiFi and LAN connectivity to all my devices, also via a managed 2.5gbit switch, IPV6 enabled

LAN3 to a smarthome controller (Tado), IPV6 disabled

LAN1 to my new Huawei B311-221, with a 4g sim in it, set in bridge mode, DHCP on FWG

The multi wan setup is in Failover, with primary connection set for LAN4 and backup in LAN1, autofailover enabled. Everything goes well, I also tried to unplug the ethernet from the ONT, FWG correctly use the backup connection with internet up&running, also the autofailover works good. This works for 2 or 3 hours, then FWG lost connection to secondary ISP. I thought it depends on 4g connection, but if I simply unplug and re-plug the ethernet cable from LAN1, it starts working again. I tried also to keep a sort of connection alive, by routing some of my devices to the 4g ISP: it lasts 6 hours, then it went down again. Any suggestion?