An IGTV link would, as it stands right now, be routed via the Generic oneboxer, which would (probably) work for the first handful of requests from any given site. After a small number of requests, IG will (probably) redirect those requests to a login screen which is what @renato is seeing.
As @renato points out, the IG onebox engine isn’t setup to handle IGTV style links. I’ve just created a PR to do this:
https://github.com/discourse/onebox/pull/446
This means the IG onebox engine will fetch the IG TV URL via the official IG oEmbed endpoint and, assuming you’ve configured an access token, the onebox URL request shouldn’t be redirected to a login screen.
(The onebox PR request will require a onebox version bump, and then an update of the Gemfile in Discourse core.)