Thay uses the Player Name + CD Key combination to determine what characters show up in your vault. So if either changes, you won't see characters in your vault anymore.
You can change your Player Name by opening up settings.tml (use Notepad) in your C:\Users\UserName\Documents\Neverwinter Nights folder, and then changing the name = "Your NWN User Name" line under [client.identity] toward the top of that file. If you are using the same CD Key that was used when you created PCs on the server with that Player Name, that is all you need to do to login and see your already existing characters.
If you are not using the same CD Key under which you initially created PCs for that Player Name, the cdkey.ini in the same folder as settings.tml is the file that holds your CD Key. Never share this file (not with another player, a server admin, your mother, your dog, or anyone else); but if you have multiple installs of NWN, you can "swap" this file from one to the other and when you login to Thay, it will then use whatever key is listed in there to validate your account along with whatever name you have specified in the settings.tml
I will caveat that I have no idea how this works on mobile though, where these files are located on mobile, and if you can even edit/swap them there, unfortunately.
Finally, while I am reluctant to do this because I don't want to open the floodgates to people requesting their character files be transferred from one account name to another -
If you feel you have a good reason that changing your user name in your settings.tml, or swapping/changing the CD Key in your cdkey.ini will not work long term (or is not working), I can move character files from one servervault to another. This isn't as big of a problem as it was in the non-NWNEE versions of Thay since scripted systems in Thay no longer rely on PC Name + Player Name to uniquely identify a PC. However, as this has not been extensively tested and is not intended, plus it is a manual process for me to do so, I do not want to make it a habit of moving characters from a servervault under one Player Name to another Player Name.
You can change your Player Name by opening up settings.tml (use Notepad) in your C:\Users\UserName\Documents\Neverwinter Nights folder, and then changing the name = "Your NWN User Name" line under [client.identity] toward the top of that file. If you are using the same CD Key that was used when you created PCs on the server with that Player Name, that is all you need to do to login and see your already existing characters.
If you are not using the same CD Key under which you initially created PCs for that Player Name, the cdkey.ini in the same folder as settings.tml is the file that holds your CD Key. Never share this file (not with another player, a server admin, your mother, your dog, or anyone else); but if you have multiple installs of NWN, you can "swap" this file from one to the other and when you login to Thay, it will then use whatever key is listed in there to validate your account along with whatever name you have specified in the settings.tml
I will caveat that I have no idea how this works on mobile though, where these files are located on mobile, and if you can even edit/swap them there, unfortunately.
Finally, while I am reluctant to do this because I don't want to open the floodgates to people requesting their character files be transferred from one account name to another -
If you feel you have a good reason that changing your user name in your settings.tml, or swapping/changing the CD Key in your cdkey.ini will not work long term (or is not working), I can move character files from one servervault to another. This isn't as big of a problem as it was in the non-NWNEE versions of Thay since scripted systems in Thay no longer rely on PC Name + Player Name to uniquely identify a PC. However, as this has not been extensively tested and is not intended, plus it is a manual process for me to do so, I do not want to make it a habit of moving characters from a servervault under one Player Name to another Player Name.