How do I start the Temple of Miraak quest?

The prerequisite quest for The Temple of Miraak is Dragonborn, which as you correctly suggest is triggered by reading the orders of the two random(ish) encounter Cultists who attack you in Skyrim. However, the prerequisite for the Dragonborn quest is not level related but requires your character to have started The Horn of Jurgen Windcaller for the Greybeards in order to be triggered.

If you visited Solstheim before starting The Horn of Jurgen Windcaller (even if you then later started and completed it) then UESP suggests that you must return to Windhelm and talk to the ship captain, Gjalund Salt-Sage, again to proceed in the quest.