call forwarding problem with AT@T phone in Europe

Hello everybody. 

I just bought a new smartphone LG V30 h931 from ebay. I live in Greece 

It is AT&T unlocked and everything works fine except for, when I try call forwarding I get a message "Call Forwarding, Connection problem or invalid MMI code" when using the MMI code . The MMI code for wind.gr (my carrier in Greece)  is **21*number_to_forward#. I also tried this with a +sign after **21 and also putting a comma at the end of the mmi code but I get the same error. I also tried with 62 instead for 21 as well as 72 but always get the same answer. 

I Tried to access call forwarding from dial menu > call settings > More > additional settings  but I get a message : "Call settings error Service unavailable". 

My android version is Oreo and software version is h93120r . 

Presumably the problem is due to the different mmi codes used by AT@T and Wind

Could someone suggest a solution?