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Is this even related to The Things Network?
Also, it’s not clear to me what you’re asking. If indeed each uplink starts with 11
then that’s not something you can fix in the decoder, but needs fixing in the device. That is: if you’re sure that 11
is not expected. Maybe you just have the wrong documentation.
Shifting only 4 bytes doesn’t look right. See Working with Bytes.
The above doesn’t look right either.