How to convert from UCS2 to UTF-8?

Question

C++ / C#

Good day to all, help me how to convert a string from an incomprehensible format to a normal form (readable).
Data comes from SerialPort from GSM modem teleofis I
read them port.ReadExisting() as a result I get the string ????\r??\nє©Hш
knowing that the modem transmits data in UCS2 format tried to convert from UTF-16 to UTF-8 without success (
Reading mode text ( = 1 ), format of received data port.Encoding = Encoding.Default;