One lovely feature of GTM is ASN based mapping. If business requires split out traffic by ISP, there's two options. One is CIDR mapping that need to register all possible resolver IPs and the other is ASN mapping that is more clear and obvious to filter specific traffics comes from. But one really inconvenience thing of ASN mapping is lack of UI support.
Here are inconveniences regarding ASN based features of GTM.
# ASN map can be created by API only
# ASN map based property can be created by API only
# Methods related ASN map need to use "Accept" or "Accept-Encoding"
# Even general methods like "List Properties" needs to be changed to use additional headers those are required for ASN based stuffs
If there's any plans to support GTM UI for ASN features, it would be really appreciated.