Adding Nav Bar to the PWA

We are trying to add a Nav Bar to the PWA. We have added custom code to the footer section but when the app loads Nav Bar appears briefly and then disappears.

Support said the following:
The app’s framework (React) may be overwriting your custom code when it updates the page. This is likely why your nav bar appears briefly and then disappears. To integrate custom elements properly, they need to be added in a way that works with our system structure.
Since custom code is not something we directly support.

Is there someone that might have an idea to achieve this differently?