You ALWAYS have to settle the batch manually at the end of the day and you need the network to be connected at that time.
The problem during the sales day is that you can't get APPROVALS via EDC if the network goes down. The fail-over router that Rob suggested will take care of this and allow you to continue processing approvals as normal. The alternative to this is:
1) Call your processor for a manual approval code - they should have given you a sticker with the phone number(s)
2) Enter the amount to be charged on the tender screen
3) When the CC Dialog comes up, enter the approval code you got in step 1 BEFORE you swipe the card
4) Swipe the card - the transaction will be inserted directly into your EDC Batch without trying to get approval, since you have already given it an approval code.
At the end of the day, or whenever your network connection is restored, settle your batch normally.