just got our T-Mobile 5G Gateway installed all TV's can load streaming movies except the one in our basement family room which never goes above 89%. What is the best booster / extender device that works with T-Mobile for streaming?

What is the "best" is a bit of a loaded question. 😎 Coverage with WIFI with the gateways is always a bit of a challenge in larger multi-story homes. I would tend to agree an Ethernet cable would probably be a good choice over an extender. If you decide to go the "extender" path just consider that you get what you pay for and extenders tend to be challenged by building materials. Another option that might be considered is using Ethernet over AC instead of drilling holes and snaking cables. A power line adapter in effect modulates the traffic from one unit to another over the electrical cable. If you buy it and it does not work out you can just return it.

TP-Link AV1000 Powerline Ethernet Adapter (~$50) TL-PA7017P KIT

The advantage of a power line adapter is that it is simple and you will not have to deal with authentication and the loss of bandwidth as with using a wireless extender. A good wireless extender is more in the neighborhood of ~$75-100 and is often more trouble to get established. The power line adapters tend to work well over the same circuit so in some instances results will vary. It is a pretty simple way to approach the problem though.