Reset your WiFi adapter, see if that helps.
Settings, System, Reset Options, Reset WiFi Mobile Bluetooth.
If your router is capable of 2.4ghz and 5ghz make sure the SSID Name for each is different, you can edit the SSID Name on the Router Admin page.
Also check your router broadcast channels for interference, change them to other channels if you find a nearby router broadcasting on the same channel as yours.
Use a WiFi analyzer to check.
I use this one from the Playstore to check for nearby routers using the same channel as me, and I switch my channel on the router to suit.
https://play.google.com/store/apps/details?id=com.vrem.wifianalyzer
Also a suggestion here for 8 Series but some of the suggestions may crossover to 6 series and others.
https://forums.oneplus.com/threads/my-findings-of-the-5-ghz-wifi-connection-issue.1221263/Click to expand...