As a matter of fact this seems different from many (if not all) failover tutorials around because of two things:
- the Mikrotik is setup as simple bridge
- the two gateways have the same IP
In my situation I briefly explored the possibility of such a setup here:
(looong topic, skip over it as it is unlikely thay it contain anything useful in your case)
At the time I also did another (crazy) experiment in GNS3 setting the Mikrotik as a bridge with three ports added to it (Lan/ISP1/ISP2) BUT disabling the ether port connected to the secondary ISP router/modem.
Then made a Netwatch script checking internet connectivity that would, in case of lost connection, disable the ether port connected to primary ISP router/modem and enable the one connected to the secondary ISP (same IP’s for both routers/modems).
To be able to check if the connectivity of ISP1 returned, I had another port (out of the bridge) also connected to the ISP1 modem/router, which was the one on which the Netwatch script was attached.
It did work, but it became complex with three ISPs (in my case) and it was “sticky” due to arp (cache) so I had to add a “gratuitous arp” script to update the arp table when the switch between ISP’s happened.
Maybe something can be built on the ashes of that.
Otherwise, all failover methods you will find will be based on the Mikrotik acting as router (i.e. natting, which in your case will likely become a double nat), if changing the router settings is not a problem, there are many ways, including the VRF based one (because of the same ISP modem/router IP) in the mentioned thread.