Track user across landing page and application

Hello, 

TL:DR
User see our add on Ticktok, directed to the landing page, download our Android app, register through Android app. How to connect events from landing page to our Android app to see the whole user journey


We need to connect our landing page to our application (web, mobile) to track the whole user journey. 
for example, on the landing page we have two events which are page_view and register_now_clicked

the user can land on our landing page so we trigger page_view event and when he clicks on register now button we trigger register_now_clicked event 

from there he is directed to our web/mobile applications to created an account and register 
The issue is that we can’t connect the user from the landing page and our apps 
we can connect the user if he register through our web app, but if he downloads our Android app and try to make any event we can’t connect him 
 

As you see here, we have more than 7k page views, around 500 clicked on register now button and only 4 registered on our app (web)
the issue is that signup event is only triggered on web platform. but there is nothing from Android and the Ads campaign is targeted to our mobile app users who can upon Ticktok ad for example and directed to google play to install the app and then try to register