The best signal I can get in house is -93dB. More realistically, it would be -95db.

With -93dB signal I can get download speeds 2--4Mbps (sometimes up to 8 Mbps). But connection is unstable.

So, I hope I can use an antenna to boost the signal from -95db to -80db. Is it realistic?

What kind of antenna? I guess It should be optimized for T-mobile 4G/HSPA+ frequencies. And, possibly, directed?

What are the frequencies for T-mobile 4G/HSPA+?

Also, what are AT commands to disable GSM/2G and force 3G/4G/HSPA on Huawei
and/or ZTE GSM modems?

For example, Huawei E366, UMG366.

Modem is connected to a CradlePoint MBR95 router, so stock connection
manager cannot be used to configure the modem to force 4G.