well if you can only BUY days with gamecards, you can use crysta for renew subscriptions. Once you've picked that option they ask you with how many days you want to renew; if you pick 90 days you will get all three veteran rewards in one go. if you pick 30 days, you will get veteran reward 1, and when your subscription is renewed you get veteran reward 2.

My question is do you need to actually redeem in mogstation or will it get sent to your character without you needing to do anything extra