Hey @Linar, thanks for your comment.
Meshtastic uses 0x2B, and I confirmed that when changing that to the default for private networks (0x12) it works also. Unfortunately we can’t change this, because it would be a breaking change.
RadioLib (which Meshtastic uses) also inserts the 0x04 for the SX126x, but the LR1110 only accepts a 1-byte sync word.