Hello @Lone_wolf 

What KPN modem do you have and if it's a Box 12, which firmware version does it have?
Is the modem the only Wifi AP or do you have extra Wifi AP's like SuperWifi (if so the white or the black ones)?

Which IP address does the Ubunu machine receive, does it also get a Gateway address and a DNS server IP address?

Does it work when using a netwerk cable?

Hello @GeSp 

My modem is KPN Box 14 (as only Wifi AP), No Internet, when tried with wired network,

It doesnot receive any IP, even though set as DHCP, Modem apparatus list shows No IP for this device and displays static,

Same machine picked up proper IP an internet connection through my Mobile hotspot and previous router from Odido

I even tried reinstalling the ubuntu OS from scratch, I noticed the even during the install I couldn't connect to KPN Wifi for some reasons.

Connecting it with a netwerk cable is not possible?

Can you force the Wifi on 2.4GHz in Ubuntu, else make an Extra Wifi network on the modem for only 2.4GHz and connect it with that SSID, does it work then?