Trouble Access DDNS within Lan

[Yoonkung]

I am having the problem access DDNS within local network, the DDNS is from Synology running directly on NAS. Everytime I reboot my router, I cannot access DDNS from within lan right away, it will take a while until I can access via DDNS. In that period of time while I cannot access via DDNS, I can access DDNS from outside the network, and I can even access with the real IP within Lan as well.
Is it normal? Is there anything to be done to gain access to DDNS within Lan right away?

My router is RT-ac5300 running the latest Merlin FW 384.6