T-Mobile 5G Home Internet Gateway slow ethernet but fast WiFi

I was seeing < 100 on my existing ethernet LAN while getting 350-400+ on the native gateway wifi.

I connected a pc directly to the TM gateway ethernet (bypassing my existing LAN) and saw fast speeds of 300+.

This pointed to an issue with the TM gateway negotiating with my existing LAN. My LAN is driven by my own wifi router.

I had changed my router to use a non-192 prefix IP address for the LAN. So I reverted this back to 192. And now my ethernet is much faster but not as fast as native. Getting 200-250, which is a lot better than < 100.

Doing a lot of testing shows the TM-gateway is throttling / load balancing between its wifi and ethernet ports. My hunch is the amount of devices on my LAN is putting some nominal load so I'm not seeing 300+ on a single device there.

Also my LAN also hosts some access points; these needed to be restarted to hit the 200-250 speeds (they were still stuck on < 75)