You could try to set additional headers
HTTP_X_FORWARDED_PROTO
(=https) and HTTP_X_FORWARDED_HOST
(=n8n.mysite.com) in IIS to see if this will help n8n with the origin.
Will probably look something like
<serverVariables>
<set name="HTTP_X_FORWARDED_PROTO" value="https" />
<set name="HTTP_X_FORWARDED_HOST" value="n8n.mysite.com" />
</serverVariables>