Hi
the issue is due to the login's SID being different on your Primary and DR instance. You can identify the SID being used on each server by using the below query.
SELECT name,sid
FROM master.sys.server_principals
You will need to recreate login on the DR instance using the exact SID details from the primary server e.g.
CREATE LOGIN mylogin WITH PASSWORD=N'mypassword', DEFAULT_DATABASE=[master], SID=0x0..