Currently, only non-dynamic url widgets are allowed in templated dashboards. That it is quite useless, as there seems to be no way to put the name of the host the template is attached to in the called url - and a templated dashboard is used to display host-specific information.
The solution is easy: add the dynamic url option to templated dashboards and auto-select the host the template is attached to as the selected host for the dashboard, so that the HOST.*-macros evaluate correctly. Should be pretty trivial.
Also see my suggestion ZBXNEXT-7937 for extended macro support in dynamic url widgets which should apply to templated dashboards as well. For a templated dashboard, template macros would need to be evaluated in addition to host and global user macros.