I saw this in the app, but when I click it changes to a $40 / 40 GB plan, still a good deal but I barely use 10GB of data a month. I have 2 lines on my account, both had the $39 / 20 GB deal, so this just makes sense. For some reason I was only able to apply to one of the lines. Reached out to Fido support on Twitter, but they weren't willing to match the deal on both my lines...
Welcome to the community!
I understand it's frustrating not being able to avail of an offer available to one of your other lines. However, the offers are exclusive to the line and not necessarily to the account (see here). I think there are many factors which are considered when identifying the offers available to a particular line. In addition to duration of service, factors likely also include usage statistics. Unfortunately, they can only apply offers available on the system for a particular line. If your other line does not have that same offer on the system, they won't be able to add it to that line.
You should also note this forum is community-driven and not intended as a venue for customer services. We would not have access to customer accounts. If you wish to discuss your situaiton, you would need to re-contact customer service as suggested above. They can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above.
Hope this helps 😀