Speed test shows way less than what my plan says

Also worth checking how the test is picking a server. Most of them choose one automatically and sometimes it grabs a busy or far away one. Switch to a nearby server and run it two or three times, single results swing a lot. And on wifi, make sure you're on the 5GHz band if your router has one. The 2.4GHz network is slower but reaches further, so phones sometimes cling to it.

First thing: are you testing over wifi? Wifi speed tests are nearly always well under the plan, especially a room or two away from the router. Plug a laptop straight into the router with a cable and test again, that's the only number you can really hold the ISP to. If it's still low wired, check the boring stuff. A lot of older laptops and some cheap routers only have 100 Mbps ethernet ports, and an old cable can drop you to that as well, so on a 300 or 500 plan you'd see a hard ceiling around 90-something. Turn off any VPN, restart the modem, and test at a couple of different times of day. If a wired test on a decent machine is still far below the plan in the morning as well as the evening, that's the point to ring the provider, and they'll usually ask you for exactly those results.