The modem installed on your M2 card doesn't exceed 500 Mbps. I've tried all versions of OpenWRT, and SW flowoff is enabled. My phone shows speeds exceeding 1 Gbps.
The modem used in the mobile phone might have better performance than RM520N-GL. Did you compare the cell bands used by mobile phones and routers? Please try to lock router to the same bands and see if it works