No, having to turn your internet off to be able to receive the OTA channels is not normal. You should be able to see all the same channels regardless of if the internet is on or off. The only thing that determines how many channels you get is the strength of the antenna (grabbing the signal) and the sensitivity of digital tuner of the tv (decoding these signals). And then of course how far away these signals are from your TV. (10 miles, 50 miles, etc)
A network router operates on the 2.4gz and 5gz frequencies. OTA channels operate at much higher frequencies (54Mhz-3000 Mhz) so these frequencies aren't even next to each other and don't conflict with each other or cause interference with each other.
If you are having to turn off internet to get the TV to display the digital sub-channels of a channel (say 11.2 and 11.3 and 11.4 etc of the main channel 11.1 ), and with the internet on, you are not seeing these same sub-channels or main channel, then there most likely exists a defect inside the TV itself regarding shielding and some internal hardware interference going on or the tuner itself is defective. Since this is not normal and defective, you really should return the TV.
Now, you also have to establish a good antenna connection before scanning for channels. You can't "tune them in" like the good old days. Once the tuner sees and acquires them during the scan, then yes, you can move your antenna around to get a better signal hopefully if weak signal. This is also why you need to periodically rescan for channels if stations add subchannels or move frequencies. (the tuner just doesn't add them to list just because they are available, you need to scan for them).
Hope that helps.
If you want to learn more than any one human being should know about antenna tv, you can check out this guy on youtube. https://www.youtube.com/watch?v=M-J5IlRyzeQ Antenna Man. He actually is very good though. In this video, he explains why some subchannels are going to be moving around soon. (not related to your situation, but helps explain subchannels.)
