You should start by visiting TG’s Manage Booking page and enter the PNR code as supplied with your confirmation email / ticket from LifeMiles. This will either work fine (sometimes, though not always, the same PNR used by the booking airline is happily recognised by the operating airline), and then you’ll be able to select seats etc ; OR will instantly throw up an error - in which case you’ll need to get hold of the PNR used by TG’s own system. To do this, you can try approaching either Avianca or Thai ……. hopefully at least one of the two will help you out.

Seems like the toughest challenge is already overcome : securing a reward seat on TG in J using LM 👍👍