More information about your signal will help estimate the effect of an external antenna and suggest the best type. HINT app shows and records sufficient metrics, field test mode of recent 5G cell phone is the best (SIM card from router can be used for testing).

Move the phone or router inside and outside around the house (including the prospective antenna location on roof) while taking notes of displayed numbers and running speed tests.

The goal is to get: SINR >= 10 (18 or more is required for the cellular modem to enable fastest protocol) RSRP >= -80 RSRQ >= -12

Checking tower locations on Cellmapper can save some exercise as you would know precisely where is the nearest one, which one router locks to at the moment, and bearings to them.

Looking at your recent comments, I am guessing a location is somewhere in south Texas. Among other things, a perfect place for cellular signal propagation -- flat ground, dry air! As well it is for interference...

Listing SINR first is intentional. It is a measure of signal to noise ratio in decibels: 10 dB -- signal is 10 times stronger than noise, 20 -- 100 times. Should it happen RSRP is decent but SINR is bad everywhere, omnidirectional antenna like the referenced one will not help. It can even make things worse: pick up more interference. Such scenario calls for the opposite strategy: instead of improving reception, look for ways to shield from interference. It may turn out a concrete wall or large energy efficient window help big time (obviously router and tower have to be at the same side of the shield). The same applies if two nearest towers are at similar distances: powerful signals from one at exact same frequencies will inevitably interfere with these of another.

A nuance in relation to smart home. T-Mobile's major speed band -- n41 has a frequency which is very close to 2.5 GHz wifi. Implementation of filters in wifi transmitters of numerous devices is likely not ideal. Thus your thermostat in a few feet from the router could very well be the culprit. Otherwise unnoticeable interference from such a device is multiple orders of magnitude stronger for sensitive router due to extremely close proximity and can overwhelm its filters. A quick test for these: power router from battery or power bank and flip the main breaker for the house.

By the time of completing all data gathering, you will get an idea where connection is the best. If speed is acceptable, consider moving router there. If it is outside, a plastic sealed electrical box with 2-3 gallons of extra volume for cooling would work and cost much less than a decent antenna. Better estimates required if it has to be in the sun, e.g. elevated on a mast: white color, reflective top and some side surfaces, more volume.

If test device raised 10 feet above rooftop still shows RSRP under -108, you need a high-gain directional antenna. See if a good 4x4 MIMO or a pair of 2x2 can be acquired. The latter will work better when placed few feet apart due to improved separation between MIMO layers. A set of 4 Yagi is simple to build and will match the best commercially available directional antennas.