No route to host when I try to open a document

I installed collabo / code on a different server, but on a subdomain of the one on which I use nexcloud.
When I try to open a document, I get this error
GuzzleHttp\Exception\ConnectException: cURL error 7: Failed to connect to underdomain.domain.tld port 9980: No route to host
But what host is he talking about? That of my nexcloud or that of collaborated on the other server?
If it is nexcloud, where to define the path to the collabora server except in the administration settings -> collabora online

I solved the problem by opening port 9980 in iptables.

iptables -A INPUT -p tcp -m tcp --dport 9980 -s -j ACCEPT