Check router speed against plan.
How much of the advertised speed you should expect depends entirely on how you tested. A wired test should reach 90 percent or more of the plan speed, because the only losses are protocol overhead and provider contention. A wifi test routinely lands at 40-70 percent even on a healthy network, since the airtime is shared and the client radio is usually the bottleneck — so judging a connection on a wifi result alone produces false alarms.
Speed against expectation by test method
Share = measured / advertised x 100; shortfall = advertised - measured; expected floor is 90 percent wired and 60 percent over wifi
Because wifi is half-duplex and shared: only one device transmits at a time, and airtime goes to whichever client can use it. A single 80 MHz 5 GHz link tops out well below a gigabit in practice even with no interference.
Fibre is often symmetric, giving a ratio near 1. Cable is typically 0.05-0.1, and DSL similar. A ratio far below the expected pattern for your technology suggests a line fault rather than a configuration issue.
Wired into the router with nothing else using the connection, repeated at different times of day. A single wifi test from a phone in another room measures your wifi, not your internet service.