OK, I’ve now got HTTPS working thanks to this guide: How To Create a SSL Certificate on Apache for Ubuntu 14.04 | DigitalOcean and now the Android app works (it needs https, it seems).
Also had to add my web address to /var/www/html/nextcloud/config/config.php:
‘trusted_domains’ =>
array (
0 => ‘192.168.1.1’,
1 => ‘my.web.address.com’,
),
Now I just need to figure out how to force it to redirect all http to https…