I have an older (~8 yrs) Quasar TV that my Roku Steaming Stick+ (3810x) remote didn't seem able to pair with to control on/off and volume. (either automatically, or by telling it that the brand was a Quasar). The help page it refers you to if that happens was really of no help. (Basically, it told you that you were SOL.)
I decided to try telling the Roku that it was another (randomly chosen) brand. After about 10 different brand attempts, it finally worked by telling the Roku that it was a Samsung TV.
So if you have a Quasar that won't pair with your Roku, try this instead. If you have a different brand TV that won't pair, try some others.