How do you use an unlocked iPhone 7 Plus with virgin mobile phone service?

I’ve tried a few times and it just keeps saying no service after searching,I’ve kept trying and once it actually showed virgin mobile with bars and Lte for less than a min but than it went to no service. This is originally a gsm phone but has been unlocked and many other people who unlocked their AT&T or t mobile iPhones were able to use it on a cdma network but why am I struggling. Any help would be much appreciated