Those settings files do not allow to add custom commands/routing tables etc. Those contain only the present defined variables. settings_ovpn.conf basically doubles the credentials, only since OpenVPN requires it with special format (username first line, password second line, nothing else).
If you need to run certain commands after VPN has been established (and before it closes), use the Edit Up + Edit Down options from dietpi-nordvpn menu.
However, the routes are and should be handled correctly by OpenVPN. 0.0.0.0/1 via 10.8.3.1 dev tun0 + 128.0.0.0/1 via 10.8.3.1 dev tun0 basically should lead to all requests being routed through the VPN, if there is no closer route defined for a certain IP range.
How is whatsmyipaddress.com resolved?
ping whatsmyipaddress.com
Ah and btw the used OpenVPN .ovpn file can be found here (as it depends on you chosen particular NordVPN server):
systemctl cat dietpi-nordvpn