The most common cause for not being able to log into the app is that the region has not been correctly selected.
The region will be the same for you and all your clients. So you will only need follow the coming steps once.
The first thing you should do is open your preferred smartphone web browser, and type in your TG URL into the search bar. I am using dmademo.telematics.guru as an example:
Circled in red, it states that the region is APAC3. Please take note of yours as it may be different.
If you open the app, you will be greeted with the login screen that will ask you to input the region, such as in the image below. Notice how the region is not currently set to APAC3 (my region), so it will not let me sign in. To change this, click on the region.
You will now be greeted with the following screen. If you select the region you took note of previously.
Log in normally.