Once your Readly Select subscription, which was given as part of the AT&T Thanks program, has ended there will be 2 options:
1.) To continue with Readly Select in paid mode; $3.99 per month (AT&T customer exclusive).
2.) To upgrade to a full Readly Premium subscription for $9.99 per month.
It's likely that Readly/AT&T will be in contact with you when your subscription is coming to an end to remind you of your options.