Hello @That_Dude, glad to see enjoy Nextcloud and self-hosting 
please work through Collabora integration guide to understand the basics of the integration.
going further into the problem: “by default” - without any additional measures - the integration often let the traffic do a loop from internal system to the internet and back.. you could avoid this using splitbraindns 101: Split-Brain DNS (split-horizon) and for more specific Docker variant look at this post Probably DNS help with NC Docker + Collabora + Wireguard tunnel