Connectivity?

I connected my new t-mobile home internet device today.

The plan details say I should get typical Download Speeds of 133-415 Mbps (5G)

I have moved this device to just about every window in the house and the best I get is around 40 Mbps with my AMPLIFIED ("Our fastest speeds with...") home internet device.

Any suggestions?  Am I missing something?

This device has the ability to utilize external antenne,  wonder if that is what need to do?

Thanks

-Shelly