Below is the firmware fix the ARP traffics leak issue that may caused the service provider modem refuse the WAN connection.
SOHO MK3:
https://download.peplink.com/firmware/br1ac/fw-max_br1mk2_hotspot_sohomk3-7.1.1s022-build1344.bin
For those that having issue that explained here, please upgrade your device using the firmware above.