Two routers in the same subnet and VPN?

What you tried to configure is doable, but inconsistent with your drawing and your requirements to use Router 2 as a boundary separating your LAN from WAN.

Router is a device which routes packets between two (or more) distinct networks.

Each of your routers must have thus at least two interfaces with IP address in separate networks.

Take a simple approach and consider each of the two routers as having LAN and WAN interfaces.

Router 1 WAN is connected to NTT
Router 1 LAN is connected to Router 2 WAN (only)
Router 2 WAN is connected to Router 1 LAN (only)
Router 2 LAN is connected to your devices

Currently your Router 2 LAN interface is in 10.0.0.0 network, which is how you access it from your other devices. You should leave it as is, so that you don't have to reconfigure the devices.

Router 2 "LAN" interface:

IP: 10.0.0.1
Netmask: 255.255.255.0
Gateway: none
DHCP server: on

Now you must configure a new network for the connection between your Router 1 WAN and Router 2 LAN.

It cannot be network 10.0.0.0/24, as you already use it on your LAN interface. /24 has the same meaning as netmask 255.255.255.0 and means the network is defined by the first three segments in dotted notation. So you must use for example 10.0.1.0/24 network.

Router 1 "LAN" interface (physically connected to the Router 2):