Unfortunately, you're experiencing what you suspect - a routing issue. These are hard to communicate to your ISP's technical support as they'll insist "all tests are fine". It will eventually get resolved - from previous posts it appears to be anywhere from 1-2 weeks. You may be able to shorten this if you're able to reach a higher escalation with support, but how to obtain that remains a mystery.
Personally, I would play via a hotspot or VPN until it is resolved as it's not worth the effort arguing with your ISP in my opinion. I don't know the magic keyword to say to get a resolution.
I believe this is an issue because EQ uses what they call "reliable UDP" :
https://www.pcgamer.com/breaking-the...ed-everything/
In the before times, when we had to wait for internet content, EQ was somewhat groundbreaking with this. Unfortunately, it has not carried over well into the modern era.
If you find out the "magic phrase" to convince your ISP's support that your traffic is experiencing a routing issue causing UDP packets to arrive out of order - please share it with us as this is a commonly reported issue.
I've been fortunate and have never run into connectivity issues. It probably has something to do with my location as I live within walking distance of numerous datacenters. I take that with a grain of salt, because the first EMPs will definitely hit here and send me back to the dark ages...
Oh yeah, and power cycle your router/modem. Can't hurt.
-Mcoy