With a bit of fiddling with aerial polarisation and channel selection I've managed to get the link working on 5GHz. RSSI is less than 40% vs 80-100% on 2.4GHz but it seems stable enough.

However, it appears that the connection is not stable in the longer term because the AP902 decides to change channels on 5GHz, seemingly at random - even though it is set to use one only. For example, yesterday I set both the AP902 and the AP900 to use channel 56. This morning the connection is down, I look at the admin pages and the AP902 says that the set channel is 56, but the active channel is 60

Back to 2.4GHz for me.