I see that the GrandStream GXP1625 IP phone is on the list of supported phones but when provisioning the phone it shows that the firmware is not supported.
I have updated the phone to the latest firmware, see below.
Please note that you have upgraded the phone the a newer firmware than the supported. The currently supported firmware is 1.0.5.125. Since this does not match with the firmware you have on the phone then you will see the warning. https://www.3cx.com/support/phone-firmwares/
We have not tested this so i cannot guarantee that everything will work but if you are not facing any issues then the management console is just a visual warning.