Getting `Could not connect to port 4444 on host` `Could not get IP address for host` error

My Jenkins and nodes are setup in AWS as EC2 instances. Jenkins host lives in the same VPC as the nodes although they live in separate subnets. The way the security groups are configured is that nodes connect to a load balancer listening on `http://custom.url:4444` which then forwards traffic to the Jenkins host. The Jenkins host is allowed to do health checks on the node using their internal IP on port 4444. Additionally, an entry to `/etc/hosts` file had to be added on Jenkins master so that nodes are able to register to this custom URL.

`0.0.0.0 custom.url`

Using this configuration, I'm able to successfully register nodes as reported both on UI and from logs. However, when I try to submit a selenium job to the grid, I get the above errors. Happens on both Chrome and Firefox browsers.

I point the tests to the custom URL in my Jenkinsfile as `pybot --variable LOCALHOST:http://custom.url:4444/wd/hub src/`.

Relevant logs from `log.html`:
```
FAIL No browser is open
DEBUG Traceback (most recent call last):
File "", line 2, in capture_page_screenshot
File "/usr/local/lib/python2.7/site-packages/Selenium2Library/keywords/keywordgroup.py", line 15, in _run_on_failure_decorator
return method(*args, **kwargs)
File "/usr/local/lib/python2.7/site-packages/Selenium2Library/keywords/_screenshot.py", line 90, in capture_page_screenshot
if hasattr(self._current_browser(), 'get_screenshot_as_file'):
File "/usr/local/lib/python2.7/site-packages/Selenium2Library/keywords/_browsermanagement.py", line 516, in _current_browser
raise RuntimeError('No browser is open')

INFO Opening browser 'firefox' to base url 'https://' through remote server at 'http://custom.url:4444/wd/hub'
INFO Could not connect to port 4444 on host custom.url
INFO Could not get IP address for host: custom.url
```
Environment:
hub: Ubuntu
nodes: Windows
packages installed on the nodes:
```
> choco list --local-only
Chocolatey v0.10.11
chocolatey 0.10.11
chocolatey-core.extension 1.3.3
chromedriver 2.410
curl 7.61.1
Firefox 62.0
git 2.19.0
git.install 2.19.0
GoogleChrome 69.0.3497.9200
javaruntime 8.0.151
jdk8 8.0.181
jre8 8.0.181
python2 2.7.15
selenium-chrome-driver 2.41
13 packages installed.
```