Not that I think it is wise to discuss optimising a terrorist activity on a public forum, though it is interesting from a threat analysis point of view.

You don't need to severe both power and data cables. For power the datacentre should be able to cope for a while, particularly if it has access to fuel deliveries. Data cable should be both easier to severe and more difficult to recover from (not the least to identify which cable has been cut where).

Most of the infrastructure of a country in term of cables run along railway tracks, sewers, etc. This means thousands of kilometers of cable even for a small country, and it is impossible to secure everything. It is impossible to severe everything either but you don't need to severe every single cable. As long as you severe enough of the backbone, the other cables will be overloaded.

So I don't think it would take that much effort for a network of terrorists to create havoc in the communications of a country for at least a few days to a week. And the consequences for the economy can be pretty dire. We have seen with BA what happens when their datacentre goes offline. Their all fleet is grounded. I imagine the consequences of a country-wide outage could be pretty dramatic. Unlikely anyone would die but you could really dent the GDP.

But if that's your methodology, you'd need to cut several cables in different locations simultaneously. A single cable doesn't take too long to fix (a couple hours, tops) and there are probably several redundant backups to handle bandwidth in the interim.