Mac can't find network after installing DSL Wi-Fi

I have just installed a new DSL wifi box but my Mac cannot find the network. It is not a problem with the network because I can connect fine on my phone and tablet. There is something wrong with the Mac.

I have tried:

Restarting Mac and router

Manually searching for the network ("the network could not be found")

I can't think what else to do nor find any more info online. Any suggestions?

[Re-Titled by Moderator]