time capsule guest network not connectable

You have to put your sky router into gateway or bridge mode. Turning its wifi off isn't enough.

 

You need the master password to the Sky router to get into the settings (Google the model number of the router). In the settings look for the bridge move toggle.

 

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That's the end of the line. Unless you can live with double-NAT. You can give it a try and if you see it'll impact your connectivity to Internet badly, you need to switch your TimeCapsule back to Bridge Mode, thus giving up the Guest network.

 

NAT stands for Network Address Translation.
Now you have 2 devices in sequence, that translate 1 external IP address to many internal IP addresses.
NAT is used, because normally your ISP will issue you just 1 public IP address (for regular home use). Now, if you have 2 or more computers at home that would like to connect to Internet - you're in trouble : you have just one address from your ISP. And the IP addresses need to be unique across all Internet, by definition. So your 2 devices can't share this address (at least use it concurrently).
That's where NAT comes in handy - it will share that 1 IP address between all your home equipment. That's also the reason your home computers get all very different IP addresses (usually in the form of 10.0.x.x or 192.168.x.x) from the one ISP assigned you.
Obviously, all your Internet partners outside will see all your connections coming from your public (ie ISP-assigned) address, not the NAT'd inside address (like 192.168.x.x).

 

Been running Double NAT on several Airports w/ no issues to speak of. I have many devices connecting to the guest network, hence guest network, of many types running Android, OSx, OS, Windows.

You can always give it a go an see how it works out. If it causes problems than switch it back.