Internet Speeds Cut by Half

Check the cellular metrics and see which bands the gateway is on. It may have changed the connections to different cells. If you check the advanced information for the cellular metrics that could explain the changes. It may have had a lock on an n41 cell source and now is on n71. That might account for the change BUT maybe not. If there are more users on the cell and there is some congestion then the speeds will be reduced as the bandwidth throttling to more evenly distribute the available bandwidth would take place.

Weather can have an impact upon the cellular communication as radio frequencies are not designed to travel through water but air. There are multiple possible factors but given the drop as you have described I would guess the signal source lock has changed. No data to know for sure and no cellular metrics before and after the change so no way to be sure.