Dear merogx !
I think you have a misunderstanding about Huawei.
The ONT used together with the OLT at the central site. The version must match the model. If the version is incorrect, it cannot work.
If you need to download the firmware, contact the sales channel partner to obtain the correct ONT firmware, and perform the upgrade or downgrade under the supervision of professional personnel. Otherwise, the fault cannot be rectified. Be careful when performing this operation.
Thank you!