I tried with free VPN but still on Play Store that Mixin is not available in Indonesia, then I tried to download the APK file from Mixin but I can't receive the 4 digit code already 6x I re-send the code it still doesn't appear via SMS.
I am able to download it with the method above. But, same as your situation, I can't receive any SMS, I changed several providers and still didn't receive it.
I use this SMS receiver online
https://temporary-phone-number.com/ to get my SMS code, which arrives a few seconds after I request a code. Probably the problem is the fact that your country is not supported by the app then use other country's SMS to receive the code for the sake of the app test.
I just want to give a thumbs up to Mixin support for being quick to solve issues encountered on Mixin Safe. I recently encounter error code 500 when I'm trying to create a safe. They fixed immediately the bug and notified me via email to continue my test. Customer support is very active in helping customers. I hope this will continue in the long term.