If your TV is one which is NO LONGER GETTING UPDATES then there’s nothing we can do except recommend you get a set top box for it. Nvidia Shield Pro, Roku Ultra, – something of that nature.
( My nephew has a 2.5 year old 4K Samsung with this problem. He grabbed the new Roku Ultra)
Regarding the certificate being UNKNOWN, Do you have your own certificate involved here somewhere? Perhaps attached / installed in the NAS itself?