Hello Rob,
RN2903 has been tested well against Multitech so it will work. I would recommend checking the RX window settings and sync word being used. All are configurable in RN2903.
I would also have suggested the channel masking but sounds like you've got that covered.
I'm assuming you are using OTAA and that the TTN software supports it in USA modes, but assumption is dangerous. Worth checking that too.
Thanks