For what it’s worth, the *#21# dialer code does work on a household phone not on Republic’s network, so, apparently, it does work in the U.S. as well as Europe. That said, it’s use is indeed not supported on Republic’s network.
No, in and of itself, this does not mean the feds or anyone else is watching you. It simply means the code does not work with Republic’s unique (sufficiently unique to be patented) blended WiFi/cell service.
Is there a particular reason, you think the feds or someone else might wish to watch you? As omnipresent as our federal government might seem at times, there are over 320 million folks living in America. It’s just not possible for the feds to meaningfully watch all of us or anything close to it.