Poor Wifi after Upgrade

I have a Mac mini late 2014. I recently replaced the standard 500gb hdd with a Samsung ssd (250gb). Everything is working fine, except the wifi. I also get occasional jumpy curser. I am using the wireless bluetooth apple keyboard and trackpad (not current versions). The other recent change was upgrading from Sierra to High Sierra. So I am not sure if the problem is hardware or software.

The wifi is usable. But is often slow or unresponsive, especially after waking from sleep. But other times it works fine. If I change settings, such as switching between 2.4 and 5ghz on my router, it seems to fix it for about a week, before the problem returns. The other thing I should say is that my router is 4g. My iPhone works without issues, so it doesn't appear to be a problem with the router. I haven't yet been able to test my Mac mini on a different network, which would obviously help diagnose the problem.

I have followed several online guides, including changing dns settings, deleting the wifi caches on my Mac, all to no avail. I have taken the Mac apart and checked the wifi antenna connection. It seems ok. The router is located about 2 or 3 metres away from the Mac, in the same room. Not sure what