(and other countries on the Balkans, it's all Huawei all-in-one GPON CPE everywhere!)
Telekom Srbija (Serbia)
SBB (Serbia)
Orion telekom (Serbia)
Supernova (Serbia)
A1 Slovenia
Telekom Slovenije (Slovenia)
Telemach (Slovenia)
A1 HR (Croatia)
Telemach (Croatia)
T-Com (Croatia)
Iskon (Croatia)
Telekom Srpske (Bosnia and Herzegovina)
Telemach (Bosnia and Herzegovina)
Triontel (Bosnia and Herzegovina)
Mtel (Montenegro)
T-Com (Montenegro)
T-Com (North Macedonia)
A1 (North Macedonia)
... and similar in Bulgaria
On all relevant websites, you cannot find info on BYO, and I can personally guarantee you cannot use your device on GPON or DOCSIS networks.
Only Romania is an exception.