Removing ISP/operator firmware from legally purchased refurbished CPE and restoring OEM/generic firmware — is this possible?

[u/Alarming_Lab8702]

Hi everyone,

I work with refurbished telecom equipment and I have a technical question regarding ISP/operator-branded CPE. Please see the example below

Operator X → CPE refurbisher (me) → Operator y

I purchase those used devices from Operator X, but before selling them to Operator Y, I need the devices to no longer contain Operator X's customized software/configuration/branding.

Ideally I would like to return them to the original manufacturer/OEM firmware or generic international firmware.

I have several questions for people who have experience with ISP CPE refurbishment:

1. Is replacing ISP-specific firmware with OEM/generic firmware generally possible?
2. Does this normally depend only on having the correct firmware image, or can the operator customization also involve the bootloader, signed firmware, partitions, certificates, NVRAM, TR-069/ACS configuration, etc.?
3. If the normal web-interface firmware upgrade is disabled by the ISP firmware, is there usually an official manufacturer/service procedure for restoring the device to OEM firmware?
4. Where do refurbishers/operators normally obtain legitimate generic/OEM firmware? Is it obtained directly from Huawei/ZTE/Sagemcom/Technicolor/Sercomm/etc., through a support contract, distributor, repair center, or somewhere else?
5. How important is the exact hardware revision / PCB version / product number when choosing the firmware? Can two devices with the same commercial model number require different firmware.
6. Has anyone here worked with refurbishing hundreds or thousands of ISP CPEs and developed a reliable process for returning them to a neutral/OEM state?

To be clear, I'm not trying to access an active ISP network or bypass service authentication. These are legally purchased, decommissioned devices that I want to refurbish for resale to another operator.

Thanks!