Switching carrier firmware
Different models use different baseband hardware and software tuned for the carrier's operation bands.

From what I found through search, a Straight Talk S5 (SM-S902L) is a Verizon S5 (SM-G900V) minus the bootloader lock, which supports LTE band 4/13. This is distinctly different from a US Cellular S5 (SM-G900R4) with LTE band 2/4/5/12/25.

Unless you've seen explicit examples that cross-flashing a baseband worked, don't even think about it.