I know AT “interfaces” from SIM modules only and I fear that you will not be able to realize the integration by adding some ATs, there must be a second layer of code that triggers the ATs. Is this open source and available, is this C or what language? I think it depents on this practical issues. In case you can program the device like an Arduino I would go this way. We have some firmware that works with this hardware base but not fully developed LoRa / TTN functionality, you may ask @Thias about details.
Btw. the Beep project used a Dragino Mini basec on an ATmega328P not Cortex M0 but code could be compatible also.