Activation of JR pass

Hi

We are coming to Japan in September and would like some advice please on what to do

We arrive at Haneda airport on 19th September - we were planning to activate our JR pass at the airport and collect a pocket WiFi at the same time. We were then going to use the JR pass to get to central Tokyo on 19th September. But my issue is we are in Japan from 19th September - 5th October so 17 days. Our first big journey using the JR pass isn't until 24th September so are we better off:
-Getting a 14 day pass and finding another way to get to central Tokyo from the airport so we then only use it from 24th September-5th October? But then if this is the case - how do we activate the pass, will we need to come back to Haneda airport to do that? Also, if we get pocket WiFi, can we get that at the airport still without the JR pass? -Or are we better off getting a 21 day pass even though we only need 17 days and its quite a bit more money?

Thanks,
Sarah