I am in the UK. Does the link point to firmware that is suitable for the UK?

Tried only one time to use previous back up & worked but if keeped bugs from previous firmwares is not recommended
Try take screenshots where the difficult settings are or write them down
If still not works at least you will have a clean firmware
Also with 3.6.4 bear in mind to change the port with SSL because is blocking other ports
http://www.draytek.net.nz/draytek/support/vigor2830-upgrading-to-3-6-4/

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Yes if fine for UK because is the annex A firmware ... that what i have with mine and is very good (apart if possible firmware bugs)

I think I've fixed this.

I had an Ethernet cable permanently connected to the 2850 which I would use on the odd occassions when I wanted a wired connection. Mostly it was lying there with nothing connected to the other end of it.

Disconnecting that Ethernet cable from the router seems to have fixed the problem.

I guess it was picking up wireless signals from the router itself and when combined with wireless signals from a neighbour the interference was enough to upset the router.