Asus Transformer Pad Infinity — WIFI connection issues with Asus ZenPad S 8.0 (P01M Z580C)

Hi all, we have a Z580c that, after being unused for a while, is having WIFI issues. Here's the situation:

Problem:

• Cannot connect to almost any network:
• Windows 10 mobile hotspot with WPA2 PSK: hangs on 'obtaining IP address'
• Android 6.01 mobile hotspot with either WPA2 PSK or no security: connects OK! (but phone has no data, so is useless)
• Fujisoft fs030w mobile router (provided/configured by Sakura mobile): no connection with either WPA2 PSK or WPS. WPS hangs on 'success' message
• Hikari Denwa RS500KI router with WPA2 PSK: no connection

My own attempts to fix:

• Have forgotten SSIDs, reset network settings
• Have done factory reset, including after installing latest firmware (WW-5.6)
• Have checked WIFI sleep settings
• Have checked time/location settings (thought it could be issue since we have moved from UK to Japan)
• Ejected SD card just in case

I'd play some more with the router settings, but the admin page is all in Japanese, which doesn't really help.

Anyone have any ideas what the problem could be?