What is your spell attack modifier when casting a spell from the Helm of the Gods?

Question

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).

Answer 1

It does say, however, that the Spell Save DC is 13 for whatever you are casting. Since Spell Save DC is normally calculated as "8 + your modifier", that would imply that your spell attack modifier would be +5. The DM, of course may choose something different, but that would be a reasonable default.

Answer 2

The Dungeon Master's Guide on spells (page 141) states:

A magic item, such as certain staffs, may require you to use your own spellcasting ability when you cast a spell from the item. If you have more than one spellcasting ability, you choose which one to use with the item. If you don't have a spellcasting ability-perhaps you're a rogue with the Use Magic Device feature- your spellcasting ability modifier is +0 for the item, and your proficiency bonus does apply.

This means the spellcasting bonus might simply be your Proficiency Bonus.

However, most items that have a fixed listed save DC also include the calculated spellcasting modifier, so it might also just be an oversight, in which case the modifier would be +5 (as stated in the answer by PhilB)