As a follow-on to this thread: Question about MTU size for very large characteristics
I'm sorry, I'm a little confused. Isn't LEATT_ATT_MTU == 23? How do you cram more bytes in there without a buffer overrun? Do you make your own BLEPROFILE_DB_PDU struct with more space and cast it?
See long_characteristic sample app in SDK 2.2.x. MTU is still 23 bytes.
The data is written by the client as a set of prepare writes followed by a commit.
Have you seen this post:
Let me know if this helps