Router Solicitation sometimes not sent

[yarn]

When I refresh the WAN connection in Interfaces - Overview, sometimes Router Solicitation is not sent, which caused the router to have no IPv6 address for itself. Why and can I fix this somehow?

My ISP expects routers to configure its address with SLAAC and request prefix through DHCPv6-PD, no address can requested via DHCPv6 or the gateway will reply with an error with no prefix given. The gateway also sends Router Advertisement very infrequently but responds to Router Solicitation.