Uncomparable slow wifi to 'normal' ethernet speed

You're right, that is a weird differential! Are you using an attached router, or are you relying on the WiFi from the gateway? I have an Arcadyan KVD21 5G gateway and get more reliable speed with an Ethernet connection, so I have things like Apple TV 4K hooked into a Netgear switch which is then plugged into the gateway, but no router. It would be normal to see a dip in download speeds during the day when Internet traffic is at its worst, but it sounds like your problem occurs consistently.

If you have a spare router you can use for a test, configure it to be in Access Point Mode, connect it to your gateway, and see if the problem remains. It's possible you have a dud of a gateway that can access the cell tower but is not working for WiFi. If the router solution corrects the problem, call Support and tell then you need a new gateway. While WiFi will always be a bit slower than Ethernet, there shouldn't be that type of differential.

¡Buena suerte!

I've noticed that if my phone has latched onto the 2.4 GHz WiFi band for some reason, rather than the 5 GHz band (I have the same SSID for both),  I can't get speeds over 80 mbps (or so).  Only if it is latched to the 5 GHz band do I get the faster speeds.  

I have Ethernet speeds from the gateway at around 225.  I have a wifi router connected over Ethernet to the gateway.  Standing next to both I get only 50 mbps using T-Mobile 5.0gz, but I get 105mbps from my router using its 5.0gz signal.  This suggests that the T-Mobile home gateway's wifi is not working up to speed.  Last week the gateways wifi speed was faster than the routers.  How do I get a different gateway?