Are you talking a coax connected antenna (for free OTA TV channels) or a true "cable TV" provided line?
If the antenna type, I'm not 100% sure - and your options will depend on the specific TV model.
If "cable TV"; I'd talk to your cable company. Typically coax will run to a cable "box" and be converted to run HDMI out to the TV.
If this is the case, it sounds like you're running an old standard (coax to TV directly?), and your provider should have modern hardware available for you now that you've moved to a "modern" TV setup.