Error 304 (Invalid PDU mode parameter) can appear at NXS-97xx 4G devices. The most common reason is that modem is misconfigured to work in 'Verizon-only' mode, so we need to switch it to 'Global' mode.
Please disable modem in the webUI (no need to reboot), then connect to your device with SSH (using root account) and issue these commands:
modem /dev/ttySMSA1 115200 at#firmware=1
modem /dev/ttySMSA1 115200 at#smsformat=0
for modem #1 in NXS-9700 4G, and also these two for modem #2 if you've NXS-9750 4G:
modem /dev/ttySMSB1 115200 at#firmware=1
modem /dev/ttySMSB1 115200 at#smsformat=0
Then please enable modem again, and try sending message - it should work.