FM11XM!! Are you sure that the device name is related to Teltonika products?
In General, before you added the domain name, Did you use the IP address of your gateway "App Server" and the device was sending payloads normally? If yes, Did you assign the server IP address to your subdomain in your Control panel of the Domain Register panel, means, Are you sure that the IP address indicates to a subdomain and vice versa?
APN is the Access Point Name of your Internet Service Provider who providing you SIM Cards with an Internet connection where it enables your device to be connected to the internet, please contact your SIM Cards provider and ask for APN settings, please be sure that your APN settings on the device are configured properly.
If the above info. is not enough for you to solve the problem, please take a snapshot of your configuration to GPRS Section or upload the device configuration file for review,
Mahmoud A.M Khalil