wifi connection

wifi is by default always searching for networks and if it finds one which is faster and has a stronger signal than the one you are connected to then it will switch to that

you can try

How to stop auto-joining a wifi network on Mac in High Sierra | iMore

and more

macos stop wifi from searching - Google

If you have not already, please use the following https://support.apple.com/en-us/HT202663 and remember the issue may be with your modem and router. The easiest and quickest thing to do is to restart both (yes even if everything else is okay) and then restart the Mac and then test.