Short answer
Do not keep trying old QR codes. First check whether the app offers an official reinstall flow.
Same phone
Open the app.
Go to Settings.
Tap Get Link to Install eSIM, if available.
Follow the reinstall steps.
New phone
If you are moving to a new phone, remove the eSIM from the old phone first, then install it on the new phone through the app.
Important
If you see an error, contact support with a screenshot. Do not try many old links/codes repeatedly.