Modem has nothing to do with how your tv picture is zoomed or its aspect ratio. You set them yourself. There's the uverse zoom (controlled from your uverse remote) and aspect ratio settings (controlled from your uverse remote menu->options->system->tv screen resolution). There's also your own tv's own zoom and resolution settings. See your user manual for that. You need to play around with some of these to get it the way you like it.
Assuming all these variables where correct before the modem change do they still look cut off to you?
Award for Community Excellence 2019 Achiever* *The views and opinions expressed on this forum are purely my own. Any product claim, statistic, quote, or other representation about a product or service should be verified with the manufacturer, provider, or party.