"Unfortunately, the process com.android.phone has stopped" error "Unfortunately, the process com.android.phone has stopped" error occurs when i try to change my network mode to LTE only.
The phone i'm using is Samsung J5(SM-J500H).
I've read that this issue will occur because of APN settings being incompatible with the settings offered by your service provider. This is specifically only if the error pops up when you try and change networks.
Try


open settings >mobile networks>network operators>select manually. Then select a network purposely that's not your service provider. Your sim will refuse to pick a signal. Revert back to select automatically. Activate then deactivate flight mode and try and select 4G network

These are other remedies that work for the error you've provided but aren't exclusively for when changing networks. They are for when the error pops up when using phone or dialer. 
Settings>Application Manager/ Applications > Switch to the All tab in Applications Manager >Telephone/Dialer/Phone 
Open the phone app and force stop>clear data>wipe cache
do the same above except instead of phone/dialer work through for sim toolkit.
Factory reset your phone. 


If all the above doesn't work then you'll have to recieve the APN settings from your provider and configure it yourself through mobile networks or have them configure it themselves.