OP seems to be stating this issue doesn’t occur w/ the 2.4GHz & the SSD connected, only w/ 5 GHz but it really does read that the SSD is throwing some EMI.
Wrap a couple of sheets of aluminium/tin foil around the SSD & its cable… but beforehand do a site survey:
Can you get into LuCI & disable the mount point? I’d like to know what happens when you’re on the 5 GHz, Repeating, initially without the SSD mounted… then what happens when you manually mount the SSD.
Having Wifi Analyzer running while doing all that should be a considerable help. You can filter results to just your specific network (in this case).
You’re going to need to SSH into your Beryl AX. See: