iPhone 6s Not Powering on

iPhone 6s was unresponsive one morning, so assuming the usual, I tried several different chargers, but the phone did not turn on again.

I tired the different options of hard reset (home button and power button together for 30s) both plugged in to USB power and also plugged into a PC. No change.

Charging had been poor so I wondered if the battery was shot - I ordered and fitted a new aftermarket battery kit (I’m reasonably comfortable taking these things apart and replacing parts). After being plugged in for over an hour, there was still no life in the phone.

Thinking that the charge might not be getting through, I also replaced the lightning port assembly (quite a faffy job!) and while the cable does now snap in really nicely, the phone still would not power on even after charging for severl hours.

I tried seeing if charge was getting in at all with a voltmeter - with the phone plugged in and the battery disconnected, there’s no voltage on the pins where the battery should attach (I don’t know if there should be though). The meter does however show that the new battery has some charge (reads 3.8v) so /should/ turn the phone on… Yeah, I know I should have tried that before changing the lightning port!

I’m staring to think there is some deeper and catastrophic hardware failure here (which I’ve honestly never seen before) but wondered if anyone here has 1) seen this set of symptoms before or 2) knows of any tricks I should still try before taking this to be looked at professionally somewhere (which might not even be possible right now due to COVID!)..

Thanks!