I have been trying to figure out the issue with my sister’s computer for a while now with no luck. It doesn’t make sense to me. My computer doesn’t have these issues on wifi or ethernet, so I am sure it is not the router. I have already reinstalled all the network drivers on her computer to no avail. I don’t know what the next step should be. Any help would be appreciated
First, update your post to include the hardware spec's and OS on the problem computer.
What network adapters are installed in her computer - make and models?
Second, check her computer to ensure that only one network adapter (either wired or wireless) is enabled. Not both. Windows 10 - try running the built in troubleshooters. They may find and fix something.
Third, if her computer is wired try another known working Ethernet cable.
What is the overall connection path to the router with respect to both wired and wireless? Network wall jacks, powerline adapters, etc. for wired connectivity.
Likewise for wireless except the parameters will be frequency, channel, distances (e.g., computer on 2nd floor, router in basement) etc..
Interference perhaps: any other wireless devices around such as baby monitors? What about neighboring wireless networks?
Also - has there been any maintenance on her computer? By that I mean the case is opened, dust, debris etc. blown out, cards and cables reseated and so forth. May be just a loose connection inside.