Well the Meraki servers don't seem to be responding to random UDP datagrams. So you can't test that way.
What you could do is setup your own UDP echo server that listens to UDP datagrams on port 9350. Then you'd need to find a way to get the router to send a datagram to your server and see if it manages to get to your server and if the response arrives too. I doubt that you can do that on Cisco routers. Another option is to have a client behind the router send a datagram.
That way at least you'll know if the ISP blocks datagrams to UDP 9350 to all IP addresses, as it's unlikely they would single out Meraki's IP addresses.
Don't forget you can always trial Meraki equipment if that's your worry.