Ideally, you should be getting the same number of channels on your RokuTV that your other non-smart TV is receiving. It seems like your antenna orientation/location is correct since you are picking up all those channels.
There previously was an issue with Roku's 10.5 OTA strength indicator that was affecting the RokuTVs but was really more a graphical bug. (showed low strength or no strength but did not affect performance of channels). This was addressed with an OS update build. I can't say if this affected the Scan feature though.
I think for troubleshooting you can try the following (before concluding an internal tv tuner issue)
1. On the RokuTV, perform a System Update to ensure you have the latest updates. (Settings..System..System Update..Check Now..[press OK]). Then do a System Restart and attempt to rescan the channels.
2. If you have another antenna available (old rabbit ears or something), try connecting those to RokuTV and see if you get additional channels. (If so, then tuner is working good and helps reinforce cabling issue or some physical connection issue.)
3. If you can, connect the coaxial cable you ran to the RokuTV to your non-smart TV. (then do the rescan and see if there is a difference--perhaps just a cabling issue.) In the alternative, you can add a coupler to the known good cable and then temporarily extend/run it down to your RokuTV and see if you can now get the additional channels after Rescanning. (Basically, move whichever is lighter, the cable or the TV).
4. The last resort may be to Factory Reset the RokuTV. (Settings..System..Advanced System Settings..Factory Reset). You will have to set your RokuTV up again but this may clear out anything that is corrupted.
Short of one of the above resolving your issue, I think it would lead me to start considering that there is an issue with the RokuTVs internal tuner.
Because of the location of my Insignia RokuTV, I just use rabbit ears on that one while all my other non-smartTVs with Roku devices are connected to my attic antenna. Short of having to sometimes adjust it for reception purposes, I believe it receives the same number of channels as my attic antenna. (or at least gets the channels I watch regularly or am interested in.)
Feel free to post back with an update or if you need more help. Some antenna enthusiasts/users in the Roku Community, so maybe others can offer you additional suggestions.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941