I wanted to post this in case it is helpful to other players having constant lag and disconnects like I was having. I have been playing on PC/NA for a couple years and have had occasional ping spikes and disconnects, sometimes seemingly related what I am doing in game (i.e. large battle in Cyrodiil, get d/c repeatedly) and other times just randomly. It never lasted very long, usually would be better the next day.
About a week and half ago I started getting pretty regular lag, ping spikes and disconnects constantly while playing, no matter what I was doing. Nothing had obviously changed on my end - I play on a laptop using wifi, cable internet with good speeds, and was getting lag/dc with noone else in the house. After a week of this I was getting pretty pissed. ESO was basically unplayable. After googling, I thought I might need to try opening ports on my router, but I do have UPNP enabled so that shouldn't be necessary (to my limited understanding).
Anyway, I noticed while looking through my Google Wifi settings that IPv6 was not enabled. I switched IPv6 to enabled and tested via a website that I was now connecting via IPv6, and tried ESO again. Ping spikes and D/C's were gone. Since then I have playing for 3 days or so with no issues. I'm not saying this will work for everybody, or even that it definitely worked for me. It could be coincidence. But it's worth a try if you're having issues like I was. As far as I can tell there's no harm, and potentially benefit to be had by enabling IPv6 on your router.