The Helm of the Gods is a magic item that, among other benefits, lets you select a spell from a list and cast it a few times a day. Unlike other items that do similar things, the Helm does not specify a spell attack modifier, and does not require attunement by a spellcaster (meaning you can't just default to your normal spell attack). Is there anything stating what, if anything, you're meant to add to the roll when casting spells from the Helm?
Whenever you finish a long rest while wearing the helm, you can pray to one of the gods listed on the Helm of the Gods table and store the listed spell in the helm, replacing any spell that is already stored there. The save DC for the spell is 13. (Mythic Odysseys of Theros, page 196).