I have a RB1100AH at both ends.
A Ubiquiti Airfiber24 link and a Ubiquiti PowerBridgeM5 link

I want to bond these two to provide link redundancy.

The primary traffic would run through the AirFiber24 link. When heavy rains hit and that link drops the traffic would move to the PowerBridgeM5 link

1. Is bonding the best solution for this redundancy setup?

2. Which mode should I use for this ? Defaults to "balance rr". "Active-backup" seems what I want but in testing on the bench failover does not seem to work.

It depends. If you need just failover, I would just run both links and play with routing to decide which link will be used for which traffic.

So use OSPF intend of bonding?

I have no experience with bonding. Maybe it is smart approach but I would expect problems when the links have different lags. If you already installed it you can just try what fits better and keep us informed.

If you do not want load balancing, only redundancy, then use bonding in "active-backup" mode. You have to set up link monitoring ARP and proper IP addresses to detect failover.