There is a unique longbow in the Waterdeep: Dragon Heist module whose description includes:
This single weapon can only be used by a creature of average size or higher with a strength of 18 or more. The bow draws oversized arrows that inflict piercing damage equivalent to 2d6 + the wearer's Force modifier …
However, looking at the PHB section on "Attacks and Dexterity Damage", I realized that it was written:
You add your Dexterity modifier to your attack roll and your damage roll when you attack with a ranged weapon …
For me, this means that, unless otherwise stated, you would add your dexterity modifier to the damage and the attack rolls of a remote weapon.
The fight with two arms is an example of prevention:
You do not add your ability modifier to bonus attack damage, unless this modifier is negative.
The NPC that owns him has a +3 skill bonus, a +7 to the big bow attacks, and deals 2d6 + 4 damage. Unfortunately, their modifiers of strength and dexterity are all four, so I can not say which ones are added.
However, as @thedarkwanderer pointed out in a commentary of this Q / A ("How can I shoot with a bow using force instead of dexterity?"):
"NPCs use different rules than PCs"
This leaves me unsure and asks the following question: if a PC uses this unique longbow, what modifiers do they add to its attack and damage rolls?