The ZEITGEIST adventure path had a similar explanation for why technology was coming to a fantasy world. One region about the size of France had been mostly stripped of magic, and a several-hundred-mile radius around it was a wild magic zone, and so industry grew up in the area.
We wanted PCs to be able to use magic on adventures, without the setting in that area having much magic. So the idea we settled on was that in the Dead Magic Zone, there's no local magic, but items from outside can carry magic for a while. It drains away within a few days, though. Innate magical abilities that refresh on a rest cannot refresh in the area, so a dragonborn gets, like, one use of their breath weapon and then has to get out of the area to regain it.
The exception to that was the 'infernal rebuke' power of tieflings, because they were created by the same curse that created the dead magic zone.
Persistent magical abilities - like a dwarf's darkvision, or a paladin's auras, or a fire elemental's entire being - would eventually be suppressed too. However, the party was never in either the dead or wild magic area for more than a few days, so we didn't have precise mechanics for this.
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To keep things fast to play, without much complexity, maybe go with:
Weak Magic. It's not possible to cast spells above level 2 in the area. Cantrips in the area have an effect as if the caster were 1st level. When you cast a spell of level 1 or 2, roll an unmodified d20 against DC 10. If the result is 9 or less, you are unable to complete the spell, but the spell slot is not expended.
Magical Power Sources. If you have a magic item, you can draw upon the magic stored in it to cast a spell. This lets you reliably cast a spell of level 1 or 2 without a chance of failure, or to cast a cantrip at your actual caster level. After you draw upon an item's magic this way, the item loses its powers until it leaves the weak magic zone.
You can also attempt to draw upon a magic item to cast a spell of level 3 to 5, which requires an unmodified d20 roll (DC 10), like for normal weak magic rules. Even drawing upon magic items, it's not possible to cast spells higher than level 5 in the weak magic area.
Limited Ambient Magic. You cannot regain spell slots above level 2 in the area.