Slow Home Internet

With the inconsistent numbers between the two routers I would question if either router is 100%

“I wanted to know if I could A) use a wifi extender to improve signal and was hoping someone here who has same issues might have tried it, or 😎 can I buy/use my own router/modem that would work better?”

A wifi extender is not going to help with the cellular signal. It will only help if the area you need to cover in your home is too large for the coverage by the Nokia wifi radios. 

The key is to get a solid cellular signal. If you do not know 100% where the cellular tower is then that is a good place to start. Confirm with T-Mobile where the tower is in relation to where your router is. They should be able to provide coordinates for the tower. The PCI for the tower would be helpful as you can confirm it is recorded in the tower data on cellmapper.net. What I found is that the PCI for the 5G NR signal was not on record in our area as a 5G tower location but the 4G LTE PCI information was available and the cell was pretty easy to identify. Data on tower locations with cellmapper.net is roughly 70-80% complete. It does contain significant data on 4G LTE towers. The data for the 5G NR equipment may or may not be updated in some locations. 

Once you know 100% where the tower is, what 5G NR band(s) T-Mobile are sending and how far out you are from that source then you can dial things in possibly. If you are out on the cell edge it might just be an exercise in futility. The reason I made the reference to the external antennas is that this can be one possible avenue to improve things. Of course if the Nokia routers you have are not solid then no external antenna can do much to help. 

To my knowledge T-Mobile does NOT support third party routers. I have seen nothing to suggest they will support the T-Mobile SIM loaded into another vendor router. Maybe someone has done it but I don't believe T-Mobile would offer any support on a non-Nokia router. In Florida you don't have large ridges and mountains as we do here so buildings could be a factor or distance to the tower. Find out how far you are from the tower and what the 5G NR bands used are then you can determine if you are  within reach of the signal with a good chance to hold it.