Could you provide more details on how you perform the test?
Are you using the same servers for both routers?
Could you attach a troubleshoot file to your question to inspect your device deeper? Make sure the issue is present before generating the file, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. The logs in the file might provide more insight into the issue.
Regarding the 5G antenna, try connecting it to the most left and most right connectors of the gateway, as these are primary transmission and reception channels for all bands, leave the middle two disconnected.
You can also view connection parameters from the router's WebUI Status -> Network -> Mobile page, to monitor mobile signal metrics, and how attaching antenna impacts the signal.