Sorry I’m a bit late to the party, but I’ve just been wresting with this issue myself, and this was one of the threads I searched. So it may be too late to help the OP, but should help those that come after.
It’s quite right that Roku don’t respond to CEC (Simplink), but LG Magic Remotes are supposed to be able to control Roku devices via Settings/All Settings/Connection/Device Connection Settings/Universal Control Settings, where you can specify a device plugged into one of the HDMI ports, and if the Magic Remote has its keypresses in its repertoire, IR or RF, it will operate it.
In fact, sometimes it will even sniff a new device and set itself up for it.
So our new LG 43” Nano just picked up the Roku Premiere I plugged into it, and worked it, straight off the bat.
But I wanted to use that downstairs, so I swapped in a Roku Streaming Stick+ instead.
No go; the LG didn’t auto-configure for it 😢
Now, after a lot of work, I have isolated the problem; the LG misrecognises the SS+, says it is configured, but isn’t. Even when I go so far into manual configuration as to be specifying OTT and then the model number, and get offered 3710X, 3810 (no X) and 3910X to choose from, it doesn’t work the SS+, a 3810X.
So in principle, LG TV Magic Remotes should work Roku devices; of the three current small devices, one works, one doesn’t, and I don’t know about the other one (the Express).
I’ll be on to LG in the morning and see what they have to say....
[Edited to get the model numbers correct]