Signal power of mobile hotspots

I have a 785S that I'm trying to improve the reliabiltiy of. I'm right at the edge of mobile coverage, with some hills between my farmhouse and the tower about 10-15 km away. It functions OK in sunny weather but overcast it can slow to almost nothing. Signal quality averages around RSRP -118 -120 dBm and -12 to -15 in bad weather. Ping can be above 60, downloads can be 7-10Mbps and uploads from zero to .30 Mbps in heavy overcast. It seems the modem doesn't have enough power to push the signal back through the cloud to the tower.

If I look into upgrading, how do I understand the power rating of the newer modems to ensure they are significantly better in ul/dl signal power. Its just a holiday house so I don't have excess $$$ to spend, so I want to be assured of real value before upgrading.