DIAGNOSIS: You've got a bottleneck somewhere in the chain. The most common culprit is an old or faulty Ethernet cable forcing a connection down to 100 Mbps instead of 1000 Mbps (Gigabit).
FIX: Let's check the wired connection first, as that rules out WiFi variables. 1) Connect a computer directly to one of the router's yellow LAN ports with an Ethernet cable. 2) Run a speed test (like speedtest.net). If you're still getting ~100 Mbps, the issue is between the modem and router. 3) Check the cable between your modem and the router's blue WAN port. It must be a Cat5e or Cat6 cable. Try a different, known-good cable. 4) Log into the router (tplinkwifi.net) and go to Advanced > Status. Under 'WAN', it should say 'Gigabit'. If it says '100Mbps', you've found the problem—it's the cable or the modem's port.
IF THAT DIDN'T WORK: For slow WiFi specifically, make sure you're connected to the 5GHz network (not 2.4GHz) and are close to the router. In the router's Wireless Settings, ensure the 5GHz 'Channel Width' is set to 80MHz, not 20 or 40MHz. A wider channel means a faster highway.
HEADS UP: Your device matters. An old phone or laptop might only have a WiFi 5 (ac) adapter, so you won't hit the absolute max WiFi 6 speeds the AX55 can offer. Also, speed tests over WiFi are best done in the same room as the router, with no walls in between.