Serial keys are intended for PC created accounts only and are unable to be redeemed on mobile.
Mobile players should purchase campaigns and add on items through the in-game store on the mobile client.
If you have purchased a Guild Wars Reforged serial key and have an existing Guild Wars account, you will need to add the serial key through the PC client.
If you have a mobile created account and do not wish to create an account for PC, you may request a refund by contacting support.