I use an RCA Universal 6 device IR remote. I found that programming it to work with external Roku devices was reasonably simple: there are three 5 digit codes for Roku devices (like the Roku stick, 3900, etc.) and one of them worked. This had to be programmed on the SAT/CBL/STREAM button on the remote. I use it because it allows me to use one remote for everything: TV, BluRay, DVR, audio amplifier, etc.; and it's also cheaper to replace than the manufactuers' remotes.
I recently bought a TCL 40S325 Roku-based TV. I tried all of the RCA codes for TCL TVs with the "TV" button on the remote, but none worked.
The solution in this case is to program the "SAT/CBL/STREAM" button on the remote to a working Roku code.
Even though your TV looks like a TV, to get an RCA Universal remote to control it you have to talk to the TV as if it was just a Roku. (In my specific case, code 54410 worked.) In a way, this does make sense, as the TV thinks it's a streaming device; though it does still also receive normal over-the-air TV channels.
In this case, TCL did something smart: although they supply a standard Roku RF remote, the TV also has a standard IR receiver (which even "blinks" when it receives a command), so making your Roku-Equipped TCL TV work with a universal IR remote is quite simple. You just need some patience as you try the available codes for the remote.
HOWEVER: I do still have one problem. The remote does not control audio volume. I don't know if it's because the TV only responds to RF signals to control remote, or if there is some better / different code that has to be entered into the universal remote.
If anyone knows how to get a Universal Remote to control both the Roku functions and the TV's volume control at the same time, please let us know.
(I was using a Pioneer Elite amplifier with the TV and programmed the RCA remote to control the volume through that: but an hour or so ago the IR in the Pioneer stopped working. I suppose I shouldn't complain too much, it is over 20 years old.)