you can not use 255.255.255.254 on a router. you have to use same subnetmask as your cable modem, This is problaby 255.255.255.248 so your ipconfig will be ip x.x.x.123 mask 255.255.255.248 gateway...

@xperiments Try to deactivate DPD, I have no good experienc with this, I have a lot off vpn to Cisco ASA and have to deactivate DPD. And not use ALL on local networks, select only 1 network, LAN for...

@dmi I do a test in my testlab with a ER605 and 4G failover on WAN/LAN1 no problem as you se..

@dmi Strange, When I do similar test it works. I have OC200 and ER7206 but no backup link. maybe TP-LINK have a answer. ? What is status off your Primary interface? is it 1000 Mbps?

Try to set primary WAN to WAN interface, not WAN/LAN1 switch the cable and restart the gateway (take the power) Maybee speedtest read only WAN

Enable Periodic Speed Test under site config.. Then the status of gateway should show