Umm... I'm not sure what 'bad and unusable' means because it could include multiple meanings.  As you might know, the mobile app works as Full VPN mode enabled so you should pick a mudfish node properly based on FAQ - Nodes

it's not working and always failed to login

Umm... Then did you try to visit http://accounts.en.loxch.com/ and change your password to simple one and test?  Sometimes it makes a problem.  :-(

seriously? password by default is made to be difficult so no one can't guess/know it, but you asking a customer to change the password to a simple one rather than fix the problem on your side?

I understand your concern but my suggestion was only for testing purpose because we're not sure that your issue is from this issue yet.  However it seems you didn't test it yet, right?

my password is simple and doesn't have any special characters even no uppercase. Still not simple yet? and still can't login via android app

Okay.. Then it seems this issue is not from your ID/PWD.  Then are you under the network firewall which could intercept your connections?