l2 gre

My ISP uses a Telrad 8101 LTE modem (I wonder if that's the correct term, modem?) which can operate as NAT/Router, L2 (GRE) bridge and L3 bridge. Yesterday I tried to set it up as L3 bridge to avoid double NAT with my router running AsusWRT Merlin 384.5 level. This option "almost" worked... I...