LAN Carp with two different WAN providers, OPENVPN clients and redundancy

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so I'm trying to use CARP on a LAN to keep clients able to access the internet through 1 of 2 opnsense gateways.

One is connected via traditional methods, other uses a 4G router.

I want the LAN to have a CARP address on it, so clients in the LAN can always hit the internet. These two devices would be physically seperate locations (sometimes seperated by radios) and using VLANs to do a WAN carp on the two devices wouldn't work out very well.

Both of these devices connect back to the main office, where they each receive an ip address respective of their COMMON NAME.

The questions are as follows:

1) Can I set this up where the two boxes will change between slave and master based upon WAN status. Doesn't appear so.

2) How can I set up the 'iroute's at the main office so that it'll fail over or should I go with openvpn tap and use OSPF across it?

3) Any other suggestions?