[Q] Huawei Ascend G525 can't access bootloader

Hello,
So I've found this old Huawei in my drawer but since it has a pattern code and I can't remember it (and it isn't registered in my google account devices), I wanted to do a hard reset of the phone.
The problem is that I can't access it. It seems to have some kind of custom firmware of my operator (Swisscom, a Swiss operator) so it always boot immediately on the main screen. I've tried the "Power button + Up volume button" and "Power button + Down volume button" but none of them worked. I've had a look at the little manual of the phone (since I still have its box) but there are no pieces of information about bootloader.
So I've tried to get rid of the pattern with adb but I don't know if USB debugging is enabled (and I can't check it since I don't know the pattern lock). So I still tried but the device is shown as "offline" in adb (and I've tried to plug the phone in different USB plugs and with different USB cables : it is still shown as offline).
So here I am with that phone I can't do anything with.

Any ideas or should I simply get rid of it ?

Thanks in advance.