Mfcttrf

Is there some official mechanic in any class, feat, spell or magic item that allows someone to use strength for ranged attacks? As an example (but opposite) The Monk gets the feature Martial Arts which allows the Monk to use dexterity instead of strength.

All I could find that might work was one specific weapon from Waterdeep

There is a weapon in the Waterdeep: Dragon Heist module, its description includes (Spoiler Alert!):

Ziraj [the Hunter]... carries an oversized longbow. This unique weapon can be used only by a Medium or larger creature that has a Strength of 18 or higher. The bow shoots oversized arrows that deal piercing damage equal to 2d6 + the wielder's Strength modifier. Its range is the same as an ordinary longbow.

Presumably this unique longbow would have its remaining stats be the same as other longbows: heavy, and two-handed, and has the ammunition property.

This longbow however does not let you use strength for your attack rolls, only your damage rolls.

This was the only method I could find to make a bow use strength.

There are other features which allow you to use a different ability scores modifiers than usual but none of them would work for you. Some example features are listed below:

Finesse weapons let you use either but no bows are finesse weapons, as shown in this table (The only ranged finesse weapon at all is the dart):

When making an attack with a finesse weapon, you use your choice of your Strength or Dexterity modifier for the attack and damage rolls...

The Hexblade Warlock's Hex Warrior feature would let you use charisma as it states:

When you attack with that weapon, you can use your Charisma modifier, instead of Strength or Dexterity, for the attack and damage rolls.

The Monk's Martial Arts feature can only replace strength with dexterity, but not the reverse as it states:

You can use Dexterity instead of Strength for the attack and damage rolls of your unarmed strikes and monk weapons.

The shillelagh spell is restricted to clubs and quarterstaves but states:

You can use your spellcasting ability instead of Strength for the attack and damage rolls of melee attacks using that weapon...

If you cannot get an this NPC's unique longbow, your best alternative is probably just using javelins; out of all the weapons that use strength they allow you to attack from the farthest away (30/120).

There is nothing official that allows Str to be used with bow attacks.

Update: Medix's answer points out the Oversized Longbow from Waterdeep which does do this, so my answer is incorrect.

None of the officially available sources (that I'm aware of) have a class feature, feat, spell, magic item or other effect that lets you add your Str modifier to bow attacks instead of Dex. If you want to be able to do this, you'll have to homebrew something like a bow with the finesse property.

Since there's nothing to cite, the reason for this not being an available option is unclear, but if I were to hazard a guess I'd say that such a feature would potentially trivialise combat; a max-str melee class with a Longbow would be able to attack either in melee or at range at full capacity with no penalties.

It would also eliminate the usefulness of Thrown Weapons, which do use your Str modifier rather than Dex for their ranged attack.