Why am I only getting 120 Mbps on my 1 Gbps fiber connection?

What exactly is the TP-Link device? It could be a switch, access point, powerline adapter, or another router, and each one could affect the result differently. The make and model of the TP-Link unit and router would help identify the limitation. If there is a wireless or powerline link anywhere in the chain, that is a likely bottleneck.

First, make sure the speed test is reporting megabits per second rather than megabytes per second. A result of 120 MB/s would be roughly 960 Mbps, which is close to a 1 Gbps connection. If the result really is 120 Mbps, temporarily connect the PC directly to the router and test again. That will show whether the TP-Link device or one of the cables is limiting the speed.

Cat 5e should handle a full gigabit connection over normal household distances, so the cable type probably isn’t the issue. Check the Ethernet adapter status on the PC and confirm the link speed is 1000/1000 Mbps, not 100/100 Mbps. Also make sure the cable is plugged into a gigabit LAN port on the router. Some TP-Link devices or router ports may only support 100 Mbps, depending on the model.

Run the test with Wi-Fi disabled on the PC and use a reputable test site, choosing a nearby server if possible. If a direct wired test still shows around 120 Mbps, check the router’s reported WAN speed and confirm the provider has activated the gigabit profile. If the direct test is fast but the current setup is slow, the TP-Link device or its connection is responsible.