@Tarun_Reddy1 , this issue is not resolved at least in firmware versions I have tried, now running RM520NGLAAR01A08M4G_01.202.01.202.
When my router is sending router solicitation message, ISP seems to send two router advertisement messages as a response but only one of those goes through to my router. ISP sends periodic router advertisement only once and this doesn’t go trough to my router causing timeout for default route and router drops it.
I have worked around this issue using rdisc6 to send extra router solicitation messages from my router.