Sandra
2 ways of doing this on Android. Sadly its not as easy as the old link between viewranger and OA.
1. Use a browser on a laptop. Log into OA using same username/password as phone. Download the GPX file from Walkhighlands and then import using the 'Route Planner' menu. You will then see it under your routes. When you next open OA on your phone it will appear.
There is a download option here - so you can hold on the phone once you get out on your walk and there is no 4G.
https://www.outdooractive.com/en/2. I use a tool on the phone called Solid Explorer. I download the GPX on the phone, and open the file using Solid. It can open the file straight into OutdoorActive. This is the easiest way.
Sadly neither is as easy as the old WH/ViewRanger link. Fingers crossed we will get that back in future.