I keep getting errors when trying to join horizon online.
My teredo is working fine and I can connect to other online games.
I tried contacting support but they closed my ticket and copy pasted an FAQ answer that was of no use.
I’m posting this as a hail mary-- I would really like to grab the Ferrari on the forzashop today…
Telling ForzaSupport “my Teredo is working fine” implies zero issues. You need to post/submit specifics: ping, generic location (country), host ISP, etc. ForzaSupport doesn’t appear to be reliable from my experience (the spotlights on the Opel Manta still don’t come on in FH3/4) but being specific usually results in a better outcome. Detailed explanations, for the most part, yield detailed results.
You’ve probably tried this, but make sure your Windows Firewall is on. Could also try pointing to a different Teredo server just in case, but if you reckon Teredo is fine then probably best not to mess with it. Best of luck 
If you could put a screenshot up or tell us the error message you are getting it would help a lot. Have you ever been able to get online in FH4? You haven’t been naughty in any way have you?