@SweetAndLow
I had this same issue, and your help worked (just wanted to mention so not wasted).
In my case it was as simple as not specifying a default gateway to my router in global configuration!
After reading this post - I specified 192.168.1.1 for namerserver and default gateway and TrueNas gave me 2 nameservers in Network Summary of the same address. It worked, then I removed the nameserver1 and just left default gateway to my router in Global Configuration as 192.168.1.1.
This then gave me one nameserver of 192.168.1.1 in network summary. Also works.
Thanks for your help in the post.