Multisite sub domain registration MU

I have setup a multi-site wordpress installation e.g. mymaindomain/subsitedomain

I’m using custom registration fields and new members receive a link to set their password.

The link takes them to the sub site domain, but when they choose a password, it directs to the maindomain with an error:
“Your password reset link appears to be invalid. Please request a new link below.”

As an administrator, I am able to give them a password and this enables them to sign-in on their subdomain.

It seems the ‘reset password’ link should not direct to the main domain and have been unable to find a solution on the forum.

My settings include:

Network settings:
The s2Member plugin has control over two options on this page.
Allow Open Registration = user and Add New Users = 1 .
Please check: **s2Member → Multisite (Config)

s2Member multisite registration configuration
Blog farm - number of sites allowed for all levels is ‘0’ (for now)
Allow open registration at level 0
All registrations are at level ‘0’ (for now)

Please can you help me troubleshoot this?