How can I find correlation between features?
Correlation is a metric that can be used when two features have the same number of total observations. Given your scenario, each router can fail for different number of times. So, we can't estimate correlation between two routers since they will be having different number of observations.

One thing you can do would be to convert every router into a time series signal, using the provided duration and start time. You can they compute the correlation then. But, it might not be really informative of the actual situation, as the signal will be predominantly sparse