Thankyou, @bruce.
As mentioned in the other thread, I've been dealing with a medical crisis, and some other matters. My apologies for such a long delay. I haven't forgotten about this thread, though.
I'm hoping that, by November, I'll have more time to work on this properly, and post some test-results. I've set reminders, on both threads, to nag me to get it done.
Thankyou for your patience, and continued good support/service.
I noticed that firmware v4.8 has been released, which I'll install (cleanly), and re-test. I'll report the results.
Ah, must it be using Uboot, specifically? Not the HTTP-UI?
Please confirm.
So far, I've been using the “online upgrade” facility within the standard/regular HTTP-UI.
I thought that Uboot was only for disaster-recovery, when the system won't boot normally.
I'm inclined to try using the (safer) normal upgrade process first. If the results are still the same, then (riskier) Uboot could be tried in future.
Thankyou. I appreciate that a return would be accepted, despite the amount of time since purchase 
. This, too, encourages me to be a repeat-customer in future, to request a replacement (not a refund), and to recommend GL·iNet to others (I know several tech-savvy people who travel often).
First, I'll upgrade the firmware and run the commands (eg iwinfo & ubus call network.wireless status) that you requested/advised. If only to help diagnose the cause of the problem (for R&D) with confidence.
Better to be sure that it really is a hardware fault, before shipping a box all the way back 
.