It happen sometimes i was unable to finish my sub, atimes due to network failure and many reasons.
I was thinking on how to put an end to it and i stumble on this idea: i did this, the night for my sub to expire, changed the date and time of my phone and remove the battrey without switching off the phone till the next day. Yes it worked!! I was able to use the sub beyond the expiring date.
Though it might not work for ya.