You seem to be certain that the red light means that communication are
absolutely and totally lost but what happens is it merely means that the
connection is intermittent or sub-optimal? What if the "red light flashy
circuit" is reacting badly?


That said, I'd certainly be badgering my ISP for an immediate (free)
replacement of the old one and the flashing LED is as good an excuse as any.

The Arris NVG510 is paid for.

I had originally understood the OP as saying there was Internet
connectivity via wi-fi to the modem but not when using Ethernet (wired)
to the modem. However, all the OP actually said is there is Internet
connectivity via wi-fi but never mentioned if there is Internet
connectivity via wired.

There is no wired connectivity. WiFi only.

Regardless of the LEDs on the modem:
- OP said wi-fi gets to Internet okay.
- No mention of whether or not wired Ethernet gets to the Internet.
- Flashing red LED doesn't match wi-fi connectivity to Internet.

From reading articles about that modem, it has a history of poor
reliability. Some users noted the modem would lose the Internet
connection and have to renegotiate several times per day. With DSL, as
I recall, there is a keep-alive ping that can be adjusted. If a reset
(not a reboot) doesn't work and if the modem is leased, exchange it.