If I attack with Rell and use ability to play Recurve Bow attached to Rell, will I get to activate Recurve Bow ability to deal 2?

No. Recurve Bow's "When I attack" trigger won't fire during the attack that played it.

Rell's trigger and Recurve Bow's trigger both need the unit to gain the Attacker designation while already having the ability. Rell is designated as the attacker first, her "When I attack" trigger fires, and Recurve Bow is only played and attached during that trigger's resolution — after the designation already happened. Attack triggers added after a unit has the Attacker designation do not process (FAQ 5019).

The bow will still work on Rell's next attack (or any later combat while attached), and it also grants the defend trigger for when Rell defends — those will fire normally since she gains the designation while already equipped.