Chris

You need satellite grade coaxial cable, with factory fitted f-type connectors on either end. Go on Amazon or somewhere similar and buy as near as possible the exact length you need to sit the VM modem where you want it, and run the cable back to the wall box.

Note that even this might not work, because altering cable length alters signal strength within your installation. If you shift the signal strength beyond tolerance you can compromise your service. VM will charge you for a call out if they have to come and fix signal levels because you’ve been doing DIY on your installation.

Personally I’d persist with the Wi-fi extender a bit first. Why isn’t it working? If your home construction makes Wi-fi signal distribution difficult, then one or more Wi-fi extenders is the logical solution. If you move the modem/router then you improve signal in one part of the house at the expense of somewhere else.