GTM and Public ISP

The answer is pretty clear,

Now considering i have the website hosted on a server located in the DR site, acting as active/active with the main site host. I think we will be doing the same basic configuration of subdomain and cname for the additional GTM. Usually the GTM can redirect the traffic from one site to another in case of server failure...... However, what happens in case the GTM is down ???? How the ISP will be able to detect the failure of the GTM ??? and would be using only a single GTM without sending any other requests to the failed GTM???? Will the user be able to notice the failure of one of the GTM ??? And for how long will the user still find the website is down ???