Are uptimekuma and whatever you’re trying to monitor on the same physical hardware, or is it all different kit?
My first feeling is that you’ve got some DNS/routing configuration that’s causing issues if you’re leaving your local network and then going through two layers before coming back in, especially if you have split horizon DNS.
The master-omnibus image bundles all that into a single container is MUCH simpler to deploy.
Literally just used their compose file they provide at https://github.com/AnalogJ/scrutiny/blob/master/docker/example.omnibus.docker-compose.yml and added in the device names and was done.