So why is this working on your synology and not on mine? What exactly did you do? Only disable the chekbox? That's all? EDIT: Works now. Had to update the synology to DSM 6.2.3-25426 Update 2 and to assign a new shared secret (for what so ever).

Disable SHA-256 compatible mode doesn’t work on my iPad Pro and on my iPhone XS (VPN on Synology). 😔