I think you may need to start by allowing traffic from your vpn-pool into the remote LAN.

Trying the same method to connect to "optimumwifi" (a WiFi mesh deployed as part of Optimum Online service). I plan to use this connection as a failover WAN link - in case building internet goes down but the local tower is still operational. I have a theory: I think the direction this need...

you have to keep RTP in mind as that is the real reason to send traffic over an alternative WAN - to dedicate a connection to improve voice quality. with that in mind you may have to add a fourth line to that config to include UDP ranges of 10000:20000, or whatever your provider's RTP port range hap...