You speak of getting a 4G modem as though that's going to significantly improve your speeds. It might and, then again, it might not.
With all due respect, the number of bars showing on your phones is a relatively poor qualitative measure of signal strength. Furthermore, most 4G phones are quite usable over a range of >50dBm (a factor of 100,000).
IMO the first thing you should do is get an absolute measure of the signal strength at your location. The Android app Network Cell Signal Lite is free and quite popular to use for this purpose.
If you measure the signal strength and find it to be less than ~-120 dBm (larger negative numbers denote weaker signal) then maybe a better phone or hotspot would provide a more advanced modem which might improve things. Or, maybe a cellular amplifier would help, but most amplifiers negate many of the technical performance improvements found in newer modems.
But please bear in mind that with digital signals signal strength and speed are only loosely related. At my home location I have a Verizon signal strength of ~-110 dBm which is really poor, but the speeds I measure are excellent.
All this is a way of saying that there may or may not be anything you can do about the speeds you are measuring. IMO there is no reason to spend money until you get a better idea of what the issue is. It simply may be that that's the best that your tower can provide.
Joel (AKA docj)--
RV Technology Specialist