There isn’t any user changeable firmware on modem only devices. Its pushed by the ISP as part of the provisioning. 

You can try disconnecting the coax from it and then holding reset for 7-15 seconds while powered on. 

I tried, then it stops the boot, lights #1 & #2 (blinking) on for a while… then I bring the coax back, then Internet is back… after 3-5 minutes it boots again.

Firmware update are completely handled only by your internet service provider. Because Arris develops the software update and makes it available to service providers.  The service providers qualify, test, and distribute the software update to devices on their network. It has limited control or visibility on distribution by the service providers.

When you’re in that 5 minute period with it blinking/working, try to log in and check the cable connections page (get a screen snip). Sometimes to high of a power can trigger a reboot

Does not give me internet anymore. It reboots right away after first light. Then four lights, then off. Then on. I tried hard reset with pin several times, no coax. Still in reboot loop. 

might file and RMA through arris if you’re still in warranty

In this case we would suggest you to reach out to arris.com/consumersupport for further assistance.