Connection to portainer server failed!

[KTac]

I am trying to set up portainer-backup as a Docker container for scheduled backups on a Raspberry Pi 4. I have changed my Portainer host name to controller. The command I am running is:

docker run \
  -d \
  --name remoteit \
  --hostname controller \
  -v $PWD/remoteit:/etc/remoteit \
  -e R3_REGISTRATION_CODE="XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX" \
  --restart unless-stopped \
  remoteit/remoteit-agent:latest

However, this is what I get when I run it:

Pinging 'controller' on the device gets a response from 127.0.0.1 as expected and Portainer is using port 9000 so I'm not sure why it can't be found. If I change the hostname value to localhost or 127.0.0.1 the error changes to connection refused.

I've set up portainer-backup before and it went very smoothly. I can't figure out why I'm running into trouble this time. Please help.