You didn’t specify vendor nor model of your TV, but this is sadly a typical example for a TV, where the Vendor stopped upgrading the firmware, so new security protocols and chipers are supported.
If there’s no firmware updates avail from the TV vendor, then there’s sadly nothing that can be done about this, and you need to look for an external player for your TV, like a Shield, Chromecast, Apple-TV or siml.
Ref: Plex support is ending 2020-09-30 on older smart TV devices
