I have 2 boxes - one that connects to the phone line (the modem) and a separate one that does WiFi (the router). I think that is the most common set-up in the US, although there are some integrated units available. The modem is provided by my ISP, but the router is mine. I did have to replace it a year or so ago because of intermittent disconnects that I couldn't trace.

Try this first: disconnect everything and power it/them down for 5 minutes. Take the opportunity (if necessary) to remove them from the alcove behind the computer and clean them thoroughly. Then power them back up (first) and reconnect them. Running the software setup again may be useful, but it did nothing for me. If that doesn't fix the problem, consider replacing the router - the modem is probably fine if you aren't having problems with hard-wired connections.