Phone won't start after charging new battery

My Moto G6, model XT1925-5 had, I thought, a failing battery. I replaced the battery, turned it on briefly to check while reinstalling, then turned it off and left it for a first charge. The charging symbol showed with a blip of the power button. Several hours later it shows no sign of life. Help!

It's an old phone, kept in use for when I want to actually look at two things at once, and because it has games where I've not worked out how to transfer scores. The batttery charge level had become wildly unreliable, sometimes turning off at 70% or working for hours while showing 0%. Recently I had the cycle of showning the first step of starting up followed by a power down followed by power up without pressing the power button, repeatedly while on charge. I decided I might as well try replacing the battery.

I took the old battery out following your guide. (I might not have used isopronal in the battery well if I'd know there were holes in it.) I cliped the new battery in (Not one of I-fixit's as out of stock, sorry) and briefly powered it on. It showed 67% (but that might be not true), connected to Wi-fi and seemed to work. I powered it down, plugged in the charger, checked that it said it was accepting charge and left it. Now it shows no sign of life.

I didn't put the back on.

What are my options for trouble shooting?

I have access to a smashed screen G6, but that is showing the same battery symptoms. My old battery pack seemed a bit puffy with air so I don't really want to put it back. Is there any point in checking the voltage of the battery pack?