DEFAULT GATEWAY IP

[#1]

@myrddin55 

Did you try 192.168.1.101 as detailed in the IPV4 table?

Try connecting locally not via a cloud based app

[#2]

@myrddin55 

This might sound like a stupid question, but WHY are you using loads of port forwarding rules with a NAS ?

Inside your LAN you SHOULD be able to access it just using the NAS's network HOSTNAME or using a fully qualified domain name(s) - (for example  'nas.myrddin55.plus.com'  or  'synology.myrddin55.plus.com'), or using it's LAN IP address.

To access your NAS remotely from elsewhere on the internet, you should be using the built-in VPN server 'package', and then ONLY forward the relevant VPN ports (e.g. UDP 1194 for OpenVPN,  or UDP 500, 1701, 4500 for IPSec VPN).

Similarly there appears to be some sort of "Mobile application" which ONLY requires TCP ports 5000 and 5001 forwarded.

You should NEVER allow other port forwards from the WAN to LAN to access a NAS - you are inviting hackers to access your files - as they will have unlimited attempts at brute force attacking your login details !.  Earlier today in an answer to somebody else, I checked MY home network firewall, and could see that hackers were port probing my connection at a rate of FIVE attacks per SECOND and my connection is invisible from the internet (so they don't know that anything is there), the rate of attack would be much higher if ANY of my network ports were visible because of unnecessary port forwards.

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[#3]

For some reason, sounds like your original static IP mapping has gone wrong, due to the different MAC address. Thoughts:

- Does your NAS have *TWO* LAN ports? Maybe the cable is in the wrong port (would tie in with the MACs being 1 different) (which model is the NAS?)

- Using a local connection from a local PC, connect to the NAS - by cable, not using Wifi. Try the old IP, try the new IP (192.168.1.101). If you can't connect, forget the router, and go back to basics....

- Did the NAS have an IP address set ON THE NAS? Or was it set by a fixed DHCP address? (both are very valid ways to allocate a fixed IP to a device)

- connect a cat5 cable directly from your PC to the NAS. Power-cycle the NAS: if set to use DHCP, I'm hoping this might cause it to revert to it's default IP, which I believe is 192.168.1.1. Set your PC to have a static IP in the same range, eg 192.168.1.2. Try to connect to the NAS on .1, .101, and whatever its original IP was. 

- what does the ethernet port LED (on NAS) show? What are the front panel LEDs doing?

And something very different: I just discovered there is a "DS Discovery" app for a phone / tablet: it will probe the local LAN to find a NAS, then let you connect to it. If it works, then if nothing else, it will show you the IP address! See this YT video: https://www.youtube.com/watch?v=VStRCQMnudI 

If you are worried about the effect of power-cycling your NAS (I'm assuming from comments that it's a Synology unit) without doing a nice software driven shutdown, don't panic. Our organisation has three Synology, and over the last 5 to 10 years that have all had unplanned power-downs (and I'm meaning 'loss of mains', rather than using front panel) and have never suffered any problems.. To shut down a Synology from the front panel, you need to press and hold the power button, for 3, 10 or 30 seconds (depends on which web page you read!)... in any case, you'll get a beep when it's accepted the button press.