Just programmed a smart key on a Audi A5 5th generation immobilizer using Lonsdor K518ISE key programmer via OBD successfully.
Not all keys lost, working key is required.
Connect Lonsdor K518 host with vehicle via ODB diagnostic socket
Select Immobilizer->AUDI->5th IMMO->Immobilizer smart key
Backup immo eeprom data before programming
Turn ignition switch on
Reading eeprom data and save it as bin format
Select Make Dealer Key
select eeprom data just read
Select A4L,A5,Q5 (no need original car key)
Insert the key to be programmed into Lonsdor K518ise key slot
Read and update current key information, then generate dealer key
Test the new smart key.