PepVPN and connection between dual B20X

I have two B20X units one at the main office and another at a branch site. The main office B20X has both cable modem WAN and T-Mobile SIM card installed so it has dual WAN connectivity. The branch however only has one WAN.

My question has to do with making sure the branch can always connect back to the main office B20X no matter what WAN the main office is currently on. Right now for the PepVPN, I have the branch connecting to main office router (branch → main office) via the static IP assigned from the cable company WAN on the main office B20X. This works fine when cable modem at the main office is up and running. However, if the cable modem goes down at the main office, I can no longer connect from the branch since (of course) the static IP is unavailable that the branch site needs to start/run the PepVPN is unavailable.

So do I need to also setup a second PepVPN from the main office to the branch (main office → branch) to make sure the PepVPN is always available at the main office site? It seems like a redundant VPN but I see no other way to do this.