Some routers and device wireless adapters have compatibility issues.These include:
1) The device cannot detect the network.
2) The device detects the network but will not connect.
3) The device connects to the network but randomly disconnects.
4) The device connects to the network, but is slower than it should be. In many cases of this fourth problem, the user might not even realise that the connection is slower than it could be.
From my experience, between 2% and 5% of device and router combinations suffer from one of these symptoms. Over the last 14 years I have had four different router/device combinations that showed problems.
1) Changing the router's wireless channel (probably will not work, but is the easiest to try).
2) Finding a new driver for the device's wireless adapter. This can be good on Windows, but more difficult with other systems, and that assumes there is a better driver available.
3) Changing the router or adding a wireless access point or WiFi Range Extender to the existing router network so that the Mac is connecting to a different WiFi source. This is best done using a different network name. I have used all three of these effectively.
4) Changing the devices wireless adapter. Easy with a Window PC, but might be more difficult with a Mac, and possibly impossible with other devices. You have to be able to connect a different wireless adapter, probably through USB, AND you must be able to install the correct driver for the new adapter.
Try the Mac on other wireless networks, which should not be too difficult if it is a laptop PC and you have some helpful friends with WiFi. If it works on other networks, then I believe a compatibility issue is the cause.
I hope this helps.