You'll want to figure out if you have a signal/modem issue or a WiFi issue. You do this by connecting directly to your modem via ethernet. If the problem goes away, you can focus on WiFi issues. If the problem remains, then look at modem related issues.
Start here: troubleshooting guide. If you still need help, please post your equipment details (make/model of your modem/gateway/router) and a screenshot of your signal levels and your error/event log. Thank you!
If it is a WiFi issue here are some tips for improving WiFi performance;
There are other options. Google for WiFi repeaters / range extenders, WiFi mesh systems / pods, powerline ethernet adapters, MoCA adapters