I have an Ultra Mobile SIM in my L5, and Mint’s settings are nearly identical; they’re practically the same company anyway, and both owned by T-mobile now:
Go to:
Settings > Mobile > Access Point Names > Create APN if you haven’t already, or if it’s not there > Fill in:
Access Points
Name Mint
APN Wholesale
Save.
Then go to:
Chats > hamburger menu > Preferences > SMS and MMS Settings > Fill in:
MMSC http://wholesale.mmsmvno.com/mms/wapenc
APN Wholesale
Apply.
You shouldn’t have to fill in anything else, and it should start working immediately.