While there is no native "one-click" untethered jailbreak for iOS 9.3.6, you can achieve an untethered state by first installing a semi-untethered jailbreak like and then applying a specific "untether" tweak from Cydia.
p0laris is a semi-untethered jailbreak for 32-bit devices running iOS 9.3. 5 and 9.3. 6. The Apple Wiki Kok3shi9 - The Apple Wiki ios 9.3 6 jailbreak untethered
: This is an untethered jailbreak for 64-bit devices running iOS 9.2–9.3.x. Common Semi-Untethered Tools While there is no native "one-click" untethered jailbreak