Is the 2nd SIM AT&T or an mvno using AT&T towers? AT&T retired their 2G network on January 1st, 2017. Whatever slot you have that is not designated as data will only access 2G. Your SIM may be looking and not finding anything...
If you need additional help or have more questions or details to share, please join the site so you can reply in this thread. See this link for instructions on how to join Windows Central.