Pushing Settings To A Device Group Is Either Stuck On "pending" Or Just Says "failed"

Hi, I have a problem,

When I push settings to a "Device group" the progress either gets stuck on pending or, it simply just fails!

I have tried the following things (so please don't reply with the following solutions):

Renew push notifications certificate / check certificate is valid.

Update to latest version of macOS and Server / check that I am on the latest version of macOS and Server.

Make sure that all required profile manager ports are open on my router / forward closed ports.

Helpful Info:

Mid - 2010 iMac with Intel i3 Processor, 1 TB hard drive.

Server 5.4

Apple Configurator is not installed and, not setup with devices in the groups and with all devices configured with profile manager on this server!

Thanks,

TechWhizz