@Kevin_Z Yes, it was the old firmware version incompatible with the Tether app, which is unsurprising. I bought the router years ago and never updated it's firmware. So, I've just updated it, and now...
@Kevin_Z Hello and thanks for your reply. I'm not sure if I understand correctly, but I believe the router should work as WiFi access point. Its my only router at home, and I have my isp cable...