Solved: Sorry, after scouring the forum more I finally understood that Load Balancing must be enabled for the Link Backup rules to work. As I only ever wanted to use one of the links at a time I thought Load Balancing should be off. It doesn't state this in the manual anywhere I could find.

I have 2 WAN connections setup, both report Link Up and Online. I created a Link Backup rule. Primary is WAN1 and backup is WAN 3, but it always selects the backup one. If I pull the cable to the backup it fails over to the primary okay, but then I plug the backup in and it switches back to that.

I tried reversing the rule, but it really likes that connection on WAN3 and keeps that selected. I cannot seem to get it to stay preferring WAN1.

They are currently both 1Gbs up/100Mbs down connections, but about to upgrade WAN1 to a 2Gbps symmetric connection, so really want to favour that one.

i can't see a mention of this in the firmware update release notes, so not keen to update just in case.