Get data from REST works on Unity but not on Android Device

Well you removed an important detail from your code which is your used URL scheme / protocol. The latest Android devices, by default, do no longer allow insecure HTTP connections and always require HTTPS. There are ways around that limitation but depending on what data you actually send / receive you might get in trouble with privacy and data security (also depends on your target countries).

Note that while using a self-signed SSL certificate could work, self-signed certificates are very often rejected by clients because they are vulnerable to man-in-the-middle attacks. If the server you want to connect to is owned by yourself you could use a Let's Encrypt certificate. Let's Encrypt certificates are usually accepted by most clients. Though if you want to be on the safe side you might want to get a proper (usually paid) certificate.

If you already use HTTPS and you are sure the certificate is configured and installed properly on your server, you should check your log when testing on your android device. Without more details we can't help you any further.

ps: I successfully use a Let's Encrypt certificate on my raspberry pi server which does re-new the certificate through a cron job every few month (since Let's Encrypt certificates have a validity of 3 months). I also have a free dyndns domain name which I created my certificate for.