Rookie Hostname/FQDN/DNS Questions

Hello Everyone.

I am struggling is with Hostname/FQDN/DNS.  I have read just about every thread with any of those words in the subject (including the zeroth rule and the DNS Best Practice thread) and I am not making any progress.  I have a DynDNS account and I have set up the account in Network Services -> DNS - > DynDNS tab and it is correctly updating my dynamic public IP.  I am able to ping my domain, but not the Sophos box. According to the instructions at Management -> System Settings -> Hostname tab:

My DynDNS hostname is "xxxx.is-a-geek.net" so in the Hostname field I entered "sophos.xxxx.is-a-geek.net".  That make this the FQDN, right?  I can ping 'xxxx.is-a-geek.net' and get a reply, but when I add 'sophos', I get 'ping request could not find sophos.xxxx.is-a-geek.net'.  What else do I need to do in order to reach the FQDN?

Ultimately, I want to be able to use hostnames inside of my network to identify devices rather than the IP address, but I need to get all this hostname/fqdn/dns stuff sorted out first - it is the zeroeth rule after all.

In case it matters, my hardware is such:

I have some other questions about allowing/blocking traffic across these interfaces, but first the hostname thing needs fixed as I believe that I need that correct in order to be able to troubleshoot further configuration.

Thank you in advance for your assistance.

A little more about my setup as it applies to the DNS Best Practice thread:

1. All of my internal networks are listed on the 'Global' tab of 'Network Services >> DNS'
2. I have read conflicting ideas about how to set this up, but I think this is right:
3. I have not set this part up yet because I do not know if my internal DNS is set up correctly
4. This is the same as 3 - not sure if my internal DNS is set up correctly, but guidance here is appreciated
5. All of my internal networks provide their own DHCP, so I just set them to point back to themselves - again guidance here is appreciated
6. I do not understand this statement
7. and 8. I do not think apply

Sorry if this is a little scattered, but I think I may be in a little over my head and I am simply trying to explain everything to the best of my ability.