One of our clients has no access to the DNS settings of the main domain, but they are developing a subdomain for this specific client.
As they have no access to the DNS settings for the main domain, they want me to use a custom subdomain that does NOT belong to the main domain / subdomain that needs to be tracked.
It is possible to deploy GTM server side on the gtm.example2.com domain. However, in this scenario, the tracking domain and the tracked domain will not be in the first-party context. As a result, some of the advanced features of the GTM server-side API will not be available. For example, it will not be possible to set a cookie, which is one of the main benefits of server-side tracking.