How can I free disk space in rescue mode

Hi,
I use VPS vps2020-starter-1-2-20:

I can't connect in normal mode anymore, I got the message / is using 99.9% of 19.2 GB.

I activated rescue mode and I mounted /dev/sdb1 disk on /mnt and I deleted some files that were taking a lot of space in my disk.
The command "df -h" shows:
/dev/sdb1 20G 16G 3.4G 83% /mnt

So it looks like I have now some space on my disk. But when I reboot my VPS on normal mode, I still can't connect and I have the same message

Do you have any solution for me please?

Thank you, have a good day!