Dual purpose port WAN & LAN in one port

[joepr]

It is possible to configure a second WAN port as dual-purpose port (WAN/LAN) and do strict fail over? I ask because I would like to have a connection agreement with my neighbor. Because of Covid I work from home. Internet outages here are common (old CATV infrastructure & unstable DOSICS concentrator). He has service with another provider (GPON). Agreed we can use each other networks during outages.

He has only one consumer grade GPON HGU router. Have planned two integrations strategies. He and I only need to connect one desktop computer. First strategy is using a VLAN.  Configure the WAN port as a LAN PORT but on a VLAN. He has an Intel NIC (vlan capable). The second option is to enable a pppoe server on the WAN port. Is any of that possible?

The connection will be made using a rather old (4-5 years) SXT 2.4 N Mikrotik CPE I have. Using the station speudobridge mode (bridge mode are not possible with a consumer AP) only one mac can be used (spseudo bridge uses the WAN  mac {automatic emulation}). Have done that in the past (to use the CPE as a ethernet to WIFI adapter) and work well (can use only one device). Thank you very much.