Connect Native Mobile App with A First Party API

Hi, there seem to be other questions related to this already but none of them make senses to me.

I have been working on an app using Flutter and trying to call the api built with NestJs(nodejs/express).

I have read the article from auth0 blog and implemented the login successfully. But if I test the access token I got through the login process raises ‘invalid signature’ on (RS256).

Also I am not sure how to call the api from the application. I have secured my api by creating a new api in auth0 dashboard and I do not know how to make the access token generated in the native application can be used in authorization in the custom api. I have found this but when I click on mobile app, it just shows how to implement login. So my question is, how do I call the first party api from my application? also, can I use the access token from the application to my api?