SIM card not working in iPhone 6

[Curler]

I have an active account using my iPhone  5.  I have a new iPhone 6 that is unlocked and my SIM card will Not work. It just keeps searching. I tried changing the APN,  But it reverts back to the old one. The APN in the iPhone 5 is isp.mb.com, but Apple says it should be sp.mb.com. for iPhone 6.
Any suggestions? I have rebooted and updated the iOS .

[gblackma]

@Curler first I would return it if possible since it no longer recives security updates. If you decide to keep it. Sync it to iTunes
1. Insert new working SIM - do not turn phone on (if SIM is already in, turn off)
2. Connect to computer with USB cable.
3. Start iTunes if it doesn't auto run
4. iPhone will switch on. Wait until iTunes registers it and checks for updates
5. Install the carrier profile update (if offered, if not, go to step 6)
6. Disconnect and restart. From @luddite.

Of this fails do a network reset and restart your phone if it doesn't automatically do so. Reset your Network Settings. Tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you've used before.

[Dunkman]

@Curler 

With iPhone 6, you normally do not need to adjust APN settings.  It is automatic.  

Maybe try network reset of the iPhone.  Toggle airplane mode on, then off.