Is the slowdown Wi-Fi or the internet connection?
A controlled wired comparison helps isolate the part of the connection that is slow. Change one condition at a time so an improvement has an interpretable cause.
What to check
Use the same device and browser. First test its normal Wi-Fi position, then near the router. If possible connect by Ethernet, disable Wi-Fi so traffic really takes the cable, and pause other transfers. Check the cable, port and adapter link speed; a slow adapter can limit the wired baseline too.
Worked example — illustrative, not a measured result
Suppose repeated results are 24 Mbps in a distant room, 140 near the router and 180 over Ethernet. The distance-dependent improvement points toward wireless coverage or interference, not proof of an ISP fault. If Ethernet and nearby Wi-Fi are both about 24, the wireless explanation is weaker.
What to do next
Move the router to an open central position, test a suitable Wi-Fi band, and avoid hiding it behind metal or placing it beside interference sources. Repeat after each change. If wired performance stays poor, compare another device and another time of day before sending the provider a dated record. Do not factory-reset a router without its setup details.