You can request a time extension so when your buyer is online again they can accept (and hopefully will), which will stop your order from being marked late. Never ever ever deliver an order that's not actually finished. That's against Fiverr's ToS.
If the buyer still won't respond, you might just want to contact customer support and see if they can cancel the order without it affecting your stats. But I'd say that's a last resort for when the buyer completely goes off grid.
Hope this helped a bit!
I'm not an expert and I'm sure the veterans of this forum can give you better advice/insights! I looked on Fiverr to see what they say about Fiverr Pro and they say this;
Taking business off of the Fiverr platform
Fiverr Pro sellers may communicate with their buyers off-platform, but please be aware that:
All off-platform communication must be documented on Fiverr to allow Fiverr to protect you in case of any buyer–seller dispute. Any attempt to take business/sales off Fiverr will result in Fiverr blocking your account, according to Fiverr’s Terms of Service.
So I guess it will be a violation if you conduct the actual business (i.e. payment) outside Fiverr. But this is for Pro Seller accounts and you don't have that, only this buyer does so that complicates it a bit too (for me haha).
In order to be safe; wouldn't it be possible to share your enthusiasm of being part of such a team, but you'd like to deliver your contributions through Fiverr? And also explaining that you prefer this approach, because Fiverr doesn't allow such communications and you don't want to risk your account?