I just registered here to mention I am on Sky Fibre in the UK using an openreach BT modem, have been using merlin with an RT-AC87U for many months, I always update to the latest firmware when it is available with no issues.
I just tried updating to 380.63 and after the upgrade I had no internet access at all across any device (the router webpage showed Internet Status Connected), I tried uploading the firmware a second time with the same result.
I didn't have time to try a factory reset and re-do all my settings, so I downgraded back to 380.62_1 and now it works again...
Sorry I cant add any logs or anything else to help but I thought this would be worth posting, it is the first time I have had an issue with a merlin firmware and had to roll back.