Load up a flash drive with DBAN. Boot from that drive. Overwrite all HDDs at least 3 times. SSDs you don’t need to do that, as a simple trim operation will do.

You are having a router issue. Specifically DHCP server on said router. Log into the routers admin page and make sure that the DHCP server is actually running.

What is happening and why do you think it is a DNS poisoning attack?

Just do a full disk encryption on the drive. Problem solved.

You should be aware that deleting a file just deletes the pointer to the file. It doesn’t over write it. That’s very important for a person who will be coding applications to be aware of.