05 intermittently NOT starting....before & after Brand new Battery

I'm Stumped. 2005 197k miles....
Thought I had a bad 12v due to never driving during this covid thing.
Purchased brand new OEM 12v AGM from dealer ($210). Installed it.
Drove it around for 10 miles or so.
parked it, turned car off.... stepped out and noticed the Smart key would not work, by either the door button and also the keyfob (no go).
Than walked to rear hatch and that smart key button also did not work.
Got back in the car and tried powering on....NOTHING.
Waited 10 seconds, tried again and it started.
What the Heck is going on??
Thoughts?

 

hey SFO - it works.....easy to change out fob battery however, still doesn't explain why vehicle won't power on when hitting the power button intermittently.

 

Thanks for that (great suggestions)...negative on both accounts (oops).
I need to run over to friends house to use his voltmeter.
I'll report back

 

MYSTERY SOLVED (Scooby doo pulled the mask off the monster to reveal)....
Bill Bixby....??
no.....we looked at the newly installed battery....
The Negative terminal wire and the ground wire were both completely loose.
(smh).
Glad this was a simple fix. Thanks for helping out!