TP-Link Archer C20 v4 - Any version - WAN speed up to 25 Mbps

When using the TP-Link Archer C20 v4 with OpenWRT, the WAN speed does not go above 25 Mbps. The modem on the other side is a Cisco DOCSIS 3.0 EPC3925 Cable modem. On TP-Link firmware, it goes up to 80 Mbps. The Internet speed (when connected to the modem) is 119 Mbps, but I do not expect more than 80-90 Mbps because of the Fast Ethernet port on the TP-Link router.

It is very unclear. How do you measure "WAN speed"?

Speed from my PC to the Internet through the WAN port, by means of Speed within LAN is appropriate (90 Mbps).