If you mean the dual logo smartthings/ADT devices that work on a proprietary protocol, there is no way to extend them, it is not a mesh network. That’s why they have a much longer range and are more reliable.
You can look at environmental issues, in particular moving the devices in the detached garage close to a clear glass window if possible, trimming bushes, stuff like that. Moving the panel indoors so that it has a path through a clear glass window on the side of the house towards the garage. But other than that there really isn’t anything you can do directly.
( garages are always tricky because they tend to have lots of metal, lots of concrete, sometimes extra insulation… If it works when the garage is empty, and doesn’t work when there are cars in it, you can try placing the devices so that they have an unobstructed line of sight to a clear glass window. )
What some people do is set up a proxy system. Put whatever devices you can that do work in the garage in the garage. When they trigger, have them trigger something inside the house that will then trigger one of the devices that can activate an ADT alert.
For example, if there is motion detected in the garage, you could turn on a fan in the house which then triggers a dual logo motion sensor in the house. But then your system for the garage has all of the vulnerabilities of the non-ADT devices, so you have to take that into account. If you can set up something that runs locally, that would help.
I’m not saying that’s a good solution, I’m just saying some people do use these kinds of proxy setups for outbuildings as being better than nothing.