OK thanks!
So I’ve learnt a bunch of stuff (steep learning curve).
Using tethering (LTE ONLY) on the SLATE I can access the ASUS Webadmin and run RDP with OpenVPN enabled. This is the behaviour I expected. It is RDP my wife needs, not internet access via OpenVPN. I must apologize here as I forgot I had “LAN only” enabled on the VPN Server; hence no internet.
I still do not know why I can NOT access the ASUS Webadmin and run RDP with OpenVPN enabled, when the SLATE is connected to the ASUS Router in repeater mode, although if it works outside my LAN it’s not a major issue. Just harder to test at home. Be nice for it work though

k.