Here you go; a right mess
You almost certainly have a voltage problem. The only thing all of this has in common is voltage source.
Ensure battery is good... even if new. Buy a load tester, they are very cheap, and load the battery.
Ensure alternator is producing 14 volts.... anything less is a problem.
Ensure you have proper grounding. Undo the grounding strap scuff it up, and reinstall.
If your alternator is bad, then it can ruin a new battery pretty quickly.
Your ecu is almost certainly okay.
Also... make sure both ends of your cables are not loose. Tight at battery, tight at starter, and tight at grounding.
Low voltage is the symptom, you need to find the cause. If you replaced the battery, or are going to, make sure to get an AGM with 850 cca minimum.