That will be related to this: System lockups on Balance 310 5G on 8.4.0 - #61 by MyCo
Enabling the watchdog timer would help, as it will reboot the primary when it crashes instead of requiring manual reboot.
Peplink have a special build of 8.5.1 that has stability improvements in it for the 310 5G/Fiber 5G and you should lodge a ticket with Peplink to get that.