I tried 3.8.9 anyway (from the dot com website) with a telnet timeout setting of 60 seconds, but sadly it still freezes after a few days. My only option at this point is to currently setup a reboot schedule to reboot every few days automatically. Not ideal but it will at least stop this happening temporarily. I haven't had a follow up from support, they asked for screenshots of the configuration but that's going to be a lot of screenshots if I screenshot everything. I asked them what they want specific screenshots of but they haven't replied for quite a few days sadly.

It's as if something is becoming full (perhaps some kind of log?) and can't save anymore data. I noticed if I send fewer commands via telnet that the router seems to last a bit longer before the interface becomes frozen. I might see about flushing all logs between telnet sessions, if all logs can be flushed.

I'm still waiting since last week for a response from technical support, which is starting to look as if it's non-existent. I tried 3.8.9 from the US website but sadly the router interface still freezes after a couple of days to a week, while the 2860 behaves perfectly fine.