First go into terminal and do arp -a to list everything on your network. If something isn’t supposed to be there that will tell you something. Kek
If all is normal then:
Unplug your router for 2 minutes then plug back in. If you have an easy WiFi password then change it. Change your DNS to 1.1.1.1 and 1.0.0.1 or whatever you prefer just never use your default dns because every ISP spies on people and tracks their web activity. Change ipv6 dns also if possible. Make sure you have a firewall on your router.
If you want to go full techfag then install wire shark and just run a packet capture. If you see a bunch of pings then you’re being Dos’d if not then there are other things you can look for like spurious data retransmissions, excessive arp packets etc
Make sure your router software is up to date. Most routers require manually updating by the user logging in. These are important because it patches security issues.
If that still don’t work then you may have damaged internet lines and should call your ISP.
You oldfags make my brain hurt