We purchased a new ONN Roku in November 2019 and we installed an outside antenna to it. We also have 3 older TVs that are not smart TVs that are connected to the same antenna. The older TVs get over 25 channels each and the Roku TV will only get 11 channels. It’s the exact same antenna. Why doesn’t my smart tv pick up the same channels as the non-smart TVs?
I can pretty much guarantee that your old TV sets are still receiving Analog channels, whereas your new TV is digital signal only.
You may find it equals out sometime soon, as the old analog transmitters break down and aren't worth fixing.
You're guaranteeing that the OP lives in a land where there are still 11 analog television signals being transmitted in her general area in 2020? If I was going to guarantee something based on so little data, I'd be far more inclined to guarantee that Walmart's ONN "house brand" of televisions is made with the cheapest components possible, including the tuner.
And/or that it might be helpful to shorten all cable runs if possible, make sure that the highest quality coaxial cable is being used, install a pre-amp as close to the antenna as possible (if one is not already being used), an RF filter if there is some kind of strong signal nearby, et cetera. Basically, if using the cheapest televisions available, it is probably a good idea to pretend that the OTA signals you wish to receive are (considerably) more distant than they actually are, and act accordingly. They're not going to bother using top-shelf tuners in televisions that seem to be chiefly intended to enable the stores to advertise "$99 specials" in their weekly ads. Because (a) they would rather you get to the store, compare that model with all the other - more expensive - ones and leave the store having purchased one of those and (b) they want to still be able to make a profit if you insist on buying the ONN. Which is fine, I suppose, since they make no great secret of this kind of thing and do a pretty good job of not trying to treat the ONN televisions as if they are anything more than they actually are. However, that does mean that the end-user must, sometimes (oft times) make allowances.