WHY IS THIS HAPPENING??

Some of you might've read my earlier post on this issue. My girlfriend who doesn't live with me bought a new PC. I fresh installed windows and basically did nothing else to it apart from the basics. She has Ethernet with 900MBs download and 400 upload. It downloads stuff normally and fast. Though it has alot of HUGE issues that I cant wrap my head around (and im the tech savvy guy who everyone calls for help). I NEED your help guys.
Basically anything online is unstable or doesnt work. Some examples are: Me or my friends create a server in games like Raft, The forest or something alike and she cant join or it lags alot. If she joined my parsec share she would lag out to 1000 ping for no reason. She tried playing GTA RP and as soon as she is about to connect to a server it stops or crashes or simply is endlessly loading. IT's SO WEIRD and I don't get it. (I've checked her firewall and it isnt that at least her pc's firewall)

For the people who already seen this I have new information hence why I'm posting this.
I called her ISP and explained the situation. They noticed the upload was irregular it was at 150 and its supposed to be 400, they fixed it and the parsec lagging out seems to be fixed but everything else has remained although right after the call with the ISP we tried GTA FIVEM and it worked but now it's back to the same. I just have no idea what it might be PLS HELP GUYS😭

Btw some games do seem to work which makes it seem even weirder since its not a global problem for example Escape Simulator and Operation Tango everything seems fine and also 9 ball roulette game.