Hi Tajinder,
We have exactly the same issue as yours in that we also access Fiori from the internet with an external name and on the inside network with an internal name. The problem is that when accessing from externally ADFS also redirects to the internal name and then the RelayState does not work due to different hostnames.
The internal hostname that ADFS redirects to comes from the Fiori metadata xml that was imported in ADFS, so ADFS will always use that to determine the hostname for redirect. From internal network access that is fine, but from external it does not work. And you can also not change the hostname when exporting the Fiori xml metadata. There is also only a single Local Provider that you can configure in the Fiori SAML2 config.
Did you ever manage to solve this issue?
Regards
Johan