Hello Chris - ok - chatted to the team.
It's unlikely our logs are going to show the information we need, but we do have some considerations that should help us both identify the issue. This happens infrequently for you - it appears to us that something might be causing your connection to go stale, so if we can identify that, we'll be able to introduce a measure to cope with it. If it's a stale connection, we may look to introduce a keep alive.
1) dynamic external IP on your home Network
Does your network provider give you a new external IP? Some broadband providers in the UK rotate the external dynamic IP once a month. Note your external IP and next time you see the issue, see if the IP has changed.
2) Next time you see this issue - on your NAS, disconnect that NAS network connection. Wait a while and reconnect the NAS to the network. Skifta will trigger a reconnect. Does this bring the device back online?
Let us know the results to these points. Your answers may assist us in improving the handling of stale connections, so thanks for being such a keen supporter of the product!