The soldering work looks pretty rough (practice on a throwaway project first). Probably one of the TRRS or MCU pins is not well connected. Get out a multimeter and do a continuity check of all the connections. It could also be misconfiguration of QMK, but make sure the basics (hardware connections) are right first.
I think this is the answer, there might be more issues with the state of the soldering but the build guide says it needs them (indicating they're using i2c for split comms).
OP needs to add 2x 4.7kOhm resistors on one half of the keyboard.