error happens when subdomain redirected to subdirectory

I have a subdomain as maindomain.subdomain.com . I forcibly redirected this subdomain to sub directory (maindomain.com/subdomain) by using some code in .htaccess .
My site works well, but I found many problems in the wordpress backend that I can’t submit a post or page. Also not sending the email through contact forms.
I found the issue that by changing the subdomain to sub directory, all submit data sends to maindomain (maindomain.com) and not to subdomain(maindomain.subdomain.com).
I should have to keep subdirectory (maindomain.com/subdomain) for SEO.
Can anybody give me a clarification?
Please note that I have no access to main domain (maindomain.com) and there is an another website run in another language and it is https.