OP dude, are you being serious??????
if you're an at&t subscriber and you have an at&t nexus... you HAVE to be registered on the network for it to even work... you just ended up manually selecting the network... no different than if you had chosen "select automatically" in the network operators panel...
has NOTHING to do with your perceived connection quality...
let's put it this way, if you were to "unregister" from the network, your device wouldn't even work... unless of course you were roaming on another network...
just enjoy your phone, stay out of the settings if you don't know what you're doing...
and any connection woes is because you're on at&t... it's an inherently $#1t network...
but hey, if you like making life more difficult for yourself, just go ahead and wipe the device clean and start over if it will make you feel better...