Use an Ethernet cable
To connect your computer to your router. Make sure you disable all radiofrequency emissions from both devices after doing this. You might want to use a radio-frequency radiation meter to check you have achieved this, as it can be hard to be sure otherwise.
RF meters vary in terms of sensitivity and frequencies measured: check with the vendor it is suitable for your use.
Hardwire your phone or tablet
You can use the internet on some phones via an Ethernet adapter. Make sure you then put it in flight mode and disable all RF emissions such as Bluetooth, GPS, Wi-Fi, 3G, 4G, 5G. You can still receive calls/texts in this mode via internet phone apps such as Skype, Whatsapp, or Facebook calling