I can't seem to configure my 2866 (the 2866K model - no WiFi or LTE) so WAN2 fails-over to WAN5.

WAN1 isn't in use.
WAN2 is connected to my ISP
WAN3/WAN4 absent
WAN5 has a Huawei E3372h-153 USB modem
WAN6: not in use

When I try to configure WAN2 to failover to WAN5, I get the message "WAN2 is already for backup".

My 'WAN >> General Setup' config settings for WAN5:

Enable: Yes
Display Name:
Physical Mode: USB
Line Speed(Kbps):
DownLink: 140
UpLink: 10
Link Condition Detection
Mode: Disable
Active Mode: Backup
Backup For: WAN 2 only selected
Active When: any of the WAN selected above: Fails to connect

Note: The USB modem (a Huawei E2272h-153) works fine in 'Always on' mode.

Failover like this worked fine on my 2862.

Maybe I'm being dim (very likely) but don't those settings for WAN5 mean that it is set to take over if WAN2 fails?

What happens if you unplug WAN2?

don't those settings for WAN5 mean that it is set to take over if WAN2 fails?

Yes they should. But the 2866 won't let me configure WAN5 to be the backup for failure of WAN2 - even though WAN2's active mode = 'A!ways on", not "Backup"! Clicking 'OK' trips the error message, and the WAN5 settings aren't saved. It does, however, let me configure WAN5 to be the backup for failure of WAN1 or WAN6.

I have raised the issue with Draytek.

Solution - use routing policy to failover - see
https://www.draytek.co.uk/support/guides/kb-wan-backup#failover-through-route-policy

1. In WAN >> General Setup, configure WAN2 and WAN5 to be always on, but not load-sharing.
2. In Routing >> Load-Balance/Route Policy, define a route policy with (index 1 so it's top-priority) which routes all packets over WAN2, select failover to WAN5, and select failback.

This configuration ensures that the 2866 only uses my backup (slow-speed 4G) mobile provider when WAN2 is down.

I still want to understand why the 2866 refuses to permit WAN5 to back-up WAN2, but lets it back-up WAN1 and WAN6.

Update: really solved it! How? As follows:
1. Logged in to my 2862 router (NOT 2866!), and *disabled* the use of WAN3 as WAN2 backup. Saved that 2862 config.
2. Logged in to 2866 (NOT 2862), and restored the 2862 config just created.
3. In WAN >> General Setup configure WAN5 to backup WAN2 (which is now accepted - yay!).

It looks as if the 2866 isn't smart enough to ignore any 2862 config settings relating to WAN3 when a 2862's config is restored to a 2866. Although the 2866's WAN ports are numbered WAN1, WAN2, WAN5 and WAN6, it seems the absent WAN3 can trip it up!

In my defence, Draytek support assured me that restoring a 2862's config to a 2866 would be fine when I was researching getting a 2866.