What permision should give to user to view application pages.

Hi,

i am using sharepoint 2019 on prem

I have created group and assign contribute permission to it,
i am using application page where i have put custom web part to allow user's to upload documents on document library.

Probelm:
as user login , he/she gets white page with progress image ( circle ), user can goto viewlsts.aspx and sitesettng.aspx using url but unable to view webparrt which is placed on application page ( its landing page )

Solution required:
What to do to solve this issue, user login to portal to find out landing page ( applicaion page with custom web part )

Note: i have not stoped/boroken inherited permission.. and if i give desginer permission, it works fine