There's actually a very simple reason for this phenomenon. A standard spring mattress essentially acts like a giant mirror, reflecting all sorts of radio waves, including your cellular signal. This can cause interference, leading to a dead zone in the near vicinity. The same effect was documented back in nineteen ninety eight when the undertaker threw mankind off hell in a cell and plummeted sixteen feet through an announcers table.
That plus those seats designed to put strain on your legs would be a 0/10 pooping experience. The only way I could think to make it worse is if you could find a way to send an all-hands message in Slack: "567primemover has been on the toilet for more than 7 minutes. Send him a word of encouragement!"
For me it's the opposite. I don't get usable mobile signal in most of my house, to the point that our phones are set up with wifi calling, and if the internet goes down (like it did for 45 minutes the other day) there's not much I can do. But if I lie down comfortably on my bed, I get a usable two bars of 4G, enough to make calls and do a bit of internet browsing.