This is likely the Baseband PMU becoming disconnected or the Audio IC. This has been labeled “Loop Disease“ and is very similar to the “Touch Disease“ issue that was on the iPhone 6 & 6+. I am a soldering specialist and I have been seeing this a ton! If you have never micro soldered before I would not start with this one. However, a re-flow may be a temporary solution but you have to be very careful with temperature as the reverse side of the board holds the baseband ic which is under-filled.
The iPhone 7 plus is very easily misaligned and either not connected or the connector itself can be damaged due to this misalignment. Step one is to test the phone with a known good battery that has at least enough charge on it to power the phone on. If it doesn’t power on with a good battery, it is likely board level. There are also a few test points on the board you can test with a multimeter to see if you are getting power through the charge port and at least to the connector.
I would honestly just re-seat the home button as it is possible to get a pressure crack and loose certain functions. If the phone was refurbished and the screen was replaced, there may not have been enough attention to detail when seating the home button.
Is WiFi grayed out or is it just not finding anything? I will say I cannot think of a time where the actual antenna itself went bad, its too simple to really fail. WiFi module or boosters on the other hand could absolutely fail.
First thing would be to take a close look at the connectors and surrounding areas for any physical damage. If you cannot see anything at all even with a clean screen and a flashlight, you may be looking at a display IC. If just the backlight goes out you can still see a faint image, but if just the display driver (u1501) or display boost (u5302_RF) goes out you can usually still see a backlight on with the screen outside of the frame. Hope this helps, but always start with the easy stuff, like looking over the board for physical marks.
Here are the liquid indicators on the mainboard and the frame. You can see the one under the sim card tray without opening the device, simply remove the sim tray and get a light in there and it will be on the frame side. There is also one on the screen if it is an OEM screen.