I don’t know anything about that; it sounds unlikely that an app would be the only way to activate service. If the phone is your only device, for instance, meaning you don’t even have home wifi, how would you download the app if your phone service isn’t yet activated? Maybe I’m misunderstanding the Mint Mobile requirement… can’t access the link at the moment because…
Sorry, you have been blocked
You are unable to access mintmobile.com
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… due to VPN? Browser privacy addons? 
See: Back to 1990 on My Librem 5 - #5 by amarok