Home internet only works with an open window/outside...

If the problem is with the coating on the windows knocking down the signals the use of an external antenna would resolve the problem and allow you to locate the gateway where you can better distribute the WIFI coverage within the home. There are a few factors to consider as the gateways do not have external connectors for external antenna connections. 

• The model of the gateway: Nokia, Arcadyan, or Sagemcon
• Technical abillity
• Cost factor
• 5G signal reception (n71 or n41)

There are some companies online like waveform.com which have antennas and guides for the connection of the antennas. They also have live technical support people that understand how things work and are very helpful in insuring your success. Waveform has a 30 day return policy as well. The guides they have are very good and clear. The older Nokia is the most simple to add the external antennas upon. One important factor is the signal received. If it is the n71 then a 2X2 MIMO is sufficient but if the frequency is the midband n41 gigahertz frequency then a 4X4 MIMO would be needed to take full advantage of the signal. A very distinct advantage of the external antenna solution is that it commonly makes a pretty distinct improvement upon the signal quality and data transfer capability due to a stronger cleaner signal. A higher quality signal will improve performance so transfer speeds are usually much better. The cost factor is a real consideration as the 2X2 MIMO is in the +$200 range and the 4X4 MIMO is +$400 to buy. Adding lightning arrestors and any other additional required cabling or mounting for unique situations has to also be considered.

Adding an external antenna also is not supported by T-Mobile and if you damage the unit when doing so well they tend to hold the user accountable for that and the cost of the gateway. Not an inexpensive proposition. The connectors for the cable dongles are small and delicate and can be broken very easily is extreme care is not taken when attachment is made. If one is broken in the process it is not the end of the world as they can be repaired and the part is not that expensive but paying a local electronic tech to do the repair can be $75-100 so obviously great care is required if the endeavor is undertaken.

The only people I can imagine that will pursue the external antenna solution are those that have no other good option and plan to stick with the solution long term and consider the investment acceptable. For an electronic hobbyist it is totally possible and attractive. It is not extreme technical but does take care and focus. If you have any remote interest check out waveform.com and have a chat with one of their expert technicians. Waveform also provided some great information via a Reddit forum, so the information is out there. You can check out other solutions online as well.