No, you can't use the effect on the linked BT21-018 [DoGatchmon] to attack. A new attack declaration can't be made during an attack. In this case, this card's [Your Turn] effect and BT21-018 [DoGatchmon]'s [When Linking] effect trigger simultaneously, and this card's [Your Turn] effect is used to attack. BT21-018 [DoGatchmon]'s [When Linking] effect can be activated before the counter timing, but because you can't declare another attack during an attack, you won't be able to attack using the linked BT21-018 [DoGatchmon]'s [When Linking] effect, even if you activate it.