Not sure how a store messed this up.. I have two nexus 5's on metropcs.. They put in the sim card... Let the phone run through its steps... Registered the sim with the IMEI... Done.. My guess is maybe your tried something..? Factory Reset? Unless someone else has another suggestion..

 

the phone is not going to take manually configured APN. by the time you back out of that menu, it will revert back to metro APN.
for some weird reason my nexus 5 needed fully 24 hours to totally settle down with calls, text, data services. maybe i mostly have weak signal from t-mob tower at my location anyway within two hours of activation i was with the metro customer service to complain about the exact problem you were describing, it dropped the call three times, never could finish the complaint. then i called from my boost phone and eventually had it done. they made me reset the phone including taking the sim card out etc. so it is better to call from another phone. its also is good idea for you to give them your imei number from settings/about phone and sim card number from your activation receipt. to make sure they have it in the system.

 

Well, how'd it go? Did it keep working?

 

Good thing it worked haha, I'm going to be connecting mine to Metro also and this sorta got me worried, but about the roaming I think just having data roaming turned on should connect you while you're roaming, but if not then I'm not sure