Thank you for the fast reply!
I found this how-to-configure failover article, which is exactly what I want except that currently both WiFi networks in our shared office are only 2.4Ghz. Maybe someday the landlord will upgrade the networks and then the fail-over solution can be self-contained in the Surf. Incidentally Cox cable has an open 5Ghz network on the devices but those require web page authentication.
Meanwhile I would have asked should I be looking at a USB adapter (if there is one that can source from WiFi) or use a WiFi-to-ethernet bridge. Do you have one that you recommend? I already have a TP-Link WR902AC that I think I can use. I had intended the Surf to fully replace the TP-Link but this solution could work for now.
BTW, I really like the Surf SOHO which seems to be the best solution I can find to this share-office scenario, especially thinking of the security benefits.