El Capitan - Losing internet connection every 20/30 minutes
If you create a new user account in macOS and that solves the problem, then you have a configuration issue or an app or daemon misbehaving.

If that does not solve the problem, based on the steps you've taken already, check to make sure your router/access point has its firmware up to date. 

If it's up to date, you might try resetting it to its defaults.