5G gateway does not work with TP-Link SG-105(un) switch - speeds reduced by about 85%

From what I can see the 5/8 port versions of the switch only have one LED indicator on the RJ-45 ports so you can’t tell for sure if it is operating at 10/100 or 1000 Mbps. With a CAT6 cable it should be good to go and the improvements with CAT6 vs CAT5e should not make that much difference. In your testing did you use more than one port on the switch for throughput testing? Maybe try different ports on the switch. Look closely at each RJ45 port and make sure the contacts are clean and in the proper orientation. If you have a second client other than the laptop you have run a throughput test between two clients to see if results are consistent or the degradation of the speed is only seen with the gateway. You don’t state which T-Mobile gateway you have. Is it the Nokia or the Arcadyan gateway? I have two different switches connected to my Nokia gateway and I have never had any significant issue. Mine are not TP-Link and mine are managed switches. If you have the Arcadyan gateway maybe you can see statistics for the ports in the mobile application. The Nokia provides statistics so it is much simpler to see if you have packet loss/damage due to TX or RX. You might load Wireshark and take a packet capture on the Ethernet port of the laptop and look for retransmissions. If there is TX or RX packet loss due to frame damage TCP retransmits would be apparent. 

Testing with two clients across the switch say with network analyzer could be helpful. It consists of a Packet Generator and a Packet Receiver/analyzer. You could get it via SourceForge.net