Hello Dave, yes, since your ISP need a VLAN ID, and VG3631 do not support this.

Seems VG3631 do not support connection with VLAN ID.

No, LAN 4 do not need to enable, however, it act as WAN port when working under EWAN mode.

Please have a check with your ISP that whether they need a "VLAN ID" when you dial up to the internet. Also, you can try mac clone on your router.

Can you post some screenshot of your log (in debug level)? Normally we can find our some reason from the log file.

This should need a function called "disabled NAT" to allow the second pppoe connection in LAN side. I am not sure whether 9977 support this function or not, please consult this problem to the support...