I dunno, but since the Threadiverse is federated, you can post on any community on lemmy.world (or elsewhere) that a federated instance can see by just creating an account on that instance.

https://piefed.fediverse.observer/

https://lemmy.fediverse.observer/

https://mbin.fediverse.observer/

I mean, there are plenty of public instances out there to choose from.

EDIT: If @dsilverz@calckey.world is correct that it's some issue that CloudFlare is taking with the exit address on the VPN, you can check whether a given instance is using CloudFlare by running whois on its IP address.

For lemmy.today:

$ host lemmy.today
lemmy.today has address 5.78.97.5
[snip]
$ whois 5.78.97.5|grep -i ^netname
netname:        CLOUD-HIL

Not using CloudFlare.

for lemmy.world:

$ host lemmy.world
lemmy.world has address 104.26.9.209
[snip]
$ whois 104.26.9.209|grep -i ^netname
NetName:        CLOUDFLARENET
$

Using CloudFlare.