Buy a better quality coax (low loss) and run from the antenna through a window to check performance. You may need a second coax from the box to the TV. If this resolves the problem then run a permanent replacement. You do not have to follow the same coax route.
Sounds like you are semi permanent. Antenna height is king. Google flag pole buddy for a easy to raise/lower an antenna, I made one from PVC. I used 2x 8' PVC pipe sections with threaded connectors for easy transport and assembly. Lots of antennas available on Amazon etc.