Unable to Connect to my SFTP Server with Public Key

Usecase

• phone A: a phone at home
  • running sshd with Termux.
• phone B: my daily phone.
  • Connected to the Internet over 5G network, or any other Wi-Fi that isn't the same network as the home one
  • trying to connect to my phone A over material files.

Key setup and test over Termux

1. On phone B, I created rsa key with Termux, and performed ssh-copy-id.
   • command used:

ssh-keygen -t rsa -b 2048 -f id_rsa
ssh-copy-id -i id_rsa.pub $username@$remote_ip_address -p $forwarded_port

1. After this, when I do ssh -p $forwarded_port $username@$remote_ip_address, it askes my key's passphrase, and ssh session starts as soon as I type it.

Material Files situation

Ahead of getting in, some names may be injust as it is shown in my home language, but not in English.

Please guess by context.

• Adding remote storage by:
  • Navigation Drawer
    • -> 'Add Storage...' which is with + icon
    • -> 'SFTP Server'
• Contents of each input fields:

Error

java8.nio.file.AccessDeniedException: /:net.schmizz.sshj.sftp.SFTPException: Permission denied

Others

Even with password authentication, it fails.
Of course,

~ $ cat ../usr/etc/ssh/sshd_config | grep "Password"
PasswordAuthentication yes

For short-term use, Termux sftp is just fine. But for long, CLI isn't the nicest solution I believe.
I want to use Material Files's beutiful GUI as my solution.

Would you help me to figure out this situation?
Thanks.

The report names Material Files' SFTP Server setup and a successful Termux SSH test, but no source files, tests, or specific code change. Start by comparing those connection details and the reported permission error; done would require a reproducible diagnosis and a defined app change, neither of which the issue currently specifies.