The router shows a speed a bit below 100Mbps through Wi-Fi 5G. Points to account for:

ISP: 300Mbps

Ethernet connection between TP-LINK Router & ISP ONT: LAN6 (verified connecting various devices directly by cable, getting full 300Mbps).

Did a full factory reset on the router, same speed problem. Checked every WLAN setting inside the ruter, the official specifications sheet and other forum threads and this device should be able to get up to 433Mbps through Wi-Fi 5G, but not on my case.