Diamond, welcome. Our first two new 86Us refused to accept an upgrade to Merlin's FW. They wouldn't save the client IP list or allow the gateway address to be changed. The third unit has had no problems. All of our devices are saved on the client IP list and routed through the router's OpenVPN clients. Nothing drops to WAN but we do have our fair share of blinking LEDs; this is the first heard of such rapid blinking.
You could test for anything hitting your address by plugging a non-critical laptop directly into the modem, running a software firewall and/or Wireshark, to log everything directed at your address. Set the laptop to sync automatically to the modem/WAN address, then repeat the test by plugging the laptop into the router port and record a new log. Turn the modem off, wait a minute or two, then power it back on and let it sync, followed by the router. Compare the logs. If you have a phone with plenty of space or a video camera, you can record the LEDs blinking during the tests, FWIW.
If the tests show no unusual incoming activity, congratulations, you may have discovered an isolated/rare bug to report to Asus. If you bought the router within the last 30 days, consider exchanging it for a new one that may not have the same set of issues. In the past Asus wasn't quick to address isolated bugs that didn't affect many units but they've been trying to improve. If you can't return your router and don't have a spare, your options are dwindling.
You might try to reinstall the Asus FW from scratch or try Merlin's FW to see if the blinking slows and could try changing your gateway address to the traditional 192.168.1.1; it can't hurt to try. Using Cloudflare or Quad9 DNS may help reduce LED rapidly-blinking syndrome (no pun intended) by cutting down on your exposure to illicit scans from bad sites, if it's not a FW bug.
If logs show you're being hammered by bad actors, as long as the addresses aren't related to your normal computing and downloading activities, consider contacting your ISP's highest technical support tier and offer your test logs to prove you're the good guy. It's always a good reason to stay on good terms with the ISP, they don't like the bad actors either. If you have the capability and resources to perform your own firewall and filtering, while it's not easy, it's your system and you're the boss. None of these chores becomes easier.
These are a couple of ideas and guesses, we haven't used the stock FW for years. Our gear runs on separate battery back-UPS and is powered up fresh daily. Not everyone can leave their system powered down at night to reduce their exposure and keep the electric bill down. Good luck, hope this helps.
