I should clarify - when I say "reset" the router I meant RESTART it. That's where I'd start, too.
After that, I might look at channel interference from other routers in your area, but I tend to doubt that it's this, to be honest, especially if you don't have trouble with other devices. Maybe look at the advanced settings for wifi on the phone (settings / wifi, tap the overflow menu bottom right, advanced settings) and turn off avoid poor connections, turn wifi frequency band to 2.4 or 5 ghz only (depending on which one your router uses), and turn off wifi optimization. Maybe not all at once, but one at a time and see if one of those improves wifi for you.
One last thing to try - see if turning off Bluetooth helps. It would kind of suck if this were the case, but I've seen more than one report of people having issues with wifi and bt when both were turned on.
I apologize if you've tried all or any of these things already - just trying to go through the obvious troubleshooting steps...