Here's the challenge... it seems the DDNS service you are using will only attempt to redirect HTTP requests.
If I type in my browser server1.atomicsparksonline.net, it attempts to connect me to the IP address you have listed there. 66.43.207.66, which times out/fails because web browser's don't 'speak' Steam protocol.
However, if I do a name server lookup on server1.atomicsparksonline.net .. it returns a totally different ip address, the one of the name server.
Perhaps it might work, but, I'm not sure it would if using steam. I'll try here in a few.
>nslookup server1.atomicsparkonline.net
Server: EyeSpyNets
Address: 192.168.1.1
Non-authoritative answer:
Name: server1.atomicsparkonline.net
Address: 67.15.250.6
A reverse lookup of that IP address shows the DDNS server, I think.
>nslookup 67.15.250.6
Server: EyeSpyNet
Address: 192.168.1.1
Name: ns1.siteground133.com
Address: 67.15.250.6