I have a AF60LR link to a customer, with a mimosa C5X ptp link running as a failover. Both links are on the same VLAN.

0 ;;; 60Ghz-LR
WAN yes yes bound 209.61.211.253/24
1 ;;; C5X
WAN2 yes yes bound 209.61.211254/24

What is the best way to setup a WAN failover in the Mikrotik 750G?

The simpler is a mikrotik device on both side and RSTP.

I have mikrotik devices on both ends but there is a trunk switch in between the first mikrotik device and the the ptp connections. Both ptp links are on the same vlan.