From what you've described sounds like an issue with DNS when onsite.
No DNS response then no way to get to tracker.moodle.org. The IP addresses you see for tracker.moodle.org should be cloudflare IP's. Cloudflare sets up their stuff for international use so a 'ray' might be configured for all traffic coming from North America and it has a name like naray.cloudflare.com For South Amercia ... saray.cloudflare.com. IF the 'ray' would would hit is having issues, you can't get to tracker. But, you say inside no DNS can find the reverse of those IP addresses and the internal DNS (inside) can't find tracker.moodle.org either.
Now the big question ... do you admin the internal DNS server? If so, check it's settings ... is it filtered to block zones? Is it's cache corrupted or confused?
If you are NOT the internal (onsite) DNS admin, request of that person to check and/or restart the DNS service ... especially IF it's Windows and they run what should be a split DNS ... contains both inside an outside IP addresss without true separation like Bind DNS does with split.
DNS server admin will probably not want to do that ... can't ever prove to those persons such issues could be due the service they administer. And they don't wanna restart nothing. That's been my experience ... hopefully not yours and you have the cooperation of that person.
'spirit of sharing', Ken