I unironically do something similar to this. In my area, the only options are a dogshit local WISP, Starlink/other satellite, or (where possible) cellular.
I am one of the “lucky” people who are able to use cell for my internet, however whether it’s the cell company having a craptastic network, software/hardware bugs on the my customer equipment, or a combination of both, there is only ONE cell tower I can connect to which yields a useful connection.
All other towers result in the equipment failing to connect to the tower, connecting but failing to get an internet connection, or only yielding download speeds 5Mbit of less.
I have found that by shoving sheet metal around my ISP’s equipment, I can quite easily block off the non-functional towers and ensure they’re never connected to. I don’t think speeds are any better, but it does help with reliability.
I have tried that. I have a dish taken from a directional WiFi antenna. When placed behind the gateway, it sometimes increases speeds, sometimes hurts speeds, and sometimes does nothing. I found it a bit too inconsistent, and a bit too ugly, to be used permanently. If I had a proper mounting solution, I might have gotten it tuned just right, however at that point I would rather just buy and mount external antennas to hook into the gateway.
My exact deployment today actually doesn’t even have anything behind the gateway. That is just because for my specific case, all the towers it can reach are within a roughly 90 degree field of view. To block the bad ones, I really only need to block off a few sections of the window it’s sitting near.