The sentence that clearly conveys the message is option 2) “we tried a new tactic to increase our roi after it had been declining for months”.
In this example, it is clear who (we) perfomed an action, what was done (tried a new tactic to increase our roi), and why the action was performed (after it had been declining for months).
The first sentence is confusing since it is not clear who or what is “declining for months”, it might have two possible meaning “we” or “our roi”.
Therefore, sentence 2) is the best option to convey a clear message.