Definitely retry. Tried upgrading my existing 4G 20GB sim only with an existing 20% student discount on on Sunday, couldn’t get the code to apply. Code from my own studentbeans account via The Open University.
Called EE on 150 on Sunday, got told the same as yourself (5G 100GB sim only deal was already on offer) so couldn’t apply the student discount code. Upgraded over the phone there and then at the full £20 cost.
After reading so many stories of people having their 20% student discount codes honoured, I called EE back on 150 this evening and asked for my student discount code to be applied. They could apply the code itself (only works during upgrade application online), but they could apply a 20% discount perk instead for the 24 month life of the 5G 100G £20 deal.
Found the EE advisor tonight to be much more helpful and accommodating that the one I spoke to on Sunday.
Still have my wife & sons lines on my account with their extra lines discount, so EE are still making £56+ for our 3 lines (mine with 5G 100GB, wife’s with 4G 20GB and sons with 4G 100GB)