That's good news - it makes sense as well that it would be an electrolytic cap causing all this trouble, I think that's main component that can wear particularly with the power supply being integrated. The Vigor 2960's firmware is generally quite reliable imo.

Please be aware that some parts of the power supply board have high voltage stored and can be very dangerous potentially. Make sure that the router is unplugged from the mains and has been left to discharge itself for at least a few minutes, if you're going to work on it. Which I cannot condone but it seems you've already done so and are hopefully aware of the risks.
The insides of the integrated PSU routers are not supposed to be worked on / replaced by end users, only at the service centre:
https://www.draytek.co.uk/support/guides/user-serviceability-of-products
With the replacement cap, make sure that the temperature rating is the same or better, a lower thermal spec than the original might work for a while but would likely have a shorter lifespan.

Capacitor replaced with one of the same spec including temperature rating, which is 105C.

Initially I thought the router board had also failed as I couldn't get it to properly start. However I realised I could get it into TFTP mode, so tried the firmware recovery utility, and followed the really detailed Draytek guide, using the .rst file to ensure everything was erased. That has worked and the 2960 is running again.

I have it set to generate an email alert if it reboots, so will monitor to see if this has fixed the problem.

As you say electrolytic capacitors are often a point of failure, and as this one had developed a 'dome' a definite sign it was failing.