Thanks for your replies.

After going directly to the ticket office only to be told "I can't select the seat. You need to call customer care" I feel frustrated. Going in circles.

What's more ridiculous is that they haven't offered to even change to an available Plus seat.

What's EVEN MORE ridiculous is that they told me I would have to pay the difference to get an Aeromexico Plus seat now that the price has increased since purchase.

I made the purchase with a debit card and the amount has already been withdrawn.