I’m afraid I really can’t remember exactly how I set all this up! it was some time ago.
If you’re sure you have the apple-app-site-association file setup correctly AND you have the required settings in your Meta data (this you need for sure to make it work) then the problem will be in x-code and the actual app. Meta data needs to look something like this…
<meta name="viewport" content="target-densitydpi=device-dpi, width=device-width, user-scalable=no, maximum-scale=1, minimum-scale=1" />
<meta name="apple-itunes-app" content="app-id=1448126601, app-argument=https://swaplot.co.uk>">
<meta property="al:ios:url" content="applinks://swaplot" />
<meta property="al:ios:app_store_id" content="1448126601" />
<meta property="al:ios:app_name" content="swapLot" />
<meta property="al:android:url" content="applinks://swaplot" />
<meta property="al:android:app_name" content="swapLot" />
<meta property="al:android:package" content="com.brommko.android.swaplot" />
<meta property="al:web:url" content="https://swaplot.co.uk" />
<link rel="manifest" href=".well-known/manifest.json">
Hopefully this will help, the above was what I used for both iOS and Android to successfully get the universal links working.