We are doing an external penetration test to our client network but can't proceed further as our scan/request are timed out every time we start the initial scan. As an example, we knew port 21 is open and could connect to it, but after we did a scan to check any available open ports, all requests(pings, etc) are timed out without any results and we can't even connect back to port 21. This would last for 10 mins and ping replies would return to normal.

We're thinking this might be an issue(or feature) on the ISP's side because all the IPs in the network block behaved in the same way. Is there any advice on how to proceed with the pentest? Should we notify the ISP or the client about the issue? Results would be very minimal and inconsistent and i am sure the client will be not satisfied if we mention the problem at the end of the day.