Thank you guys for providing the help, I tried and haven't been able to figure it all out, so glad I still learned something, you've been truly amazing!
New interface, static address 10.0.0.1, connected to SSID Pi, IPV4 gateway set to 192.168.1.139, DHCP enabled for subnet 10.0.0.x;
New firewall group for allowing communications from and to "LAN", where 192.168.1.139 belongs.
The problem:
Devices connected to "Pi" don't recognise 10.0.0.1 as network gateway, devices are assigned 10.0.0.x IPs though, and setting manually the network gateway doesn't help either. What did I do wrong?
It is not so clear what are you trying to achieve here.
The RPi is a gateway to what exactly? From the diagram the only gateway is the OpenWrt to the internet. The rest looks like hosts to the OpenWrt lan.
Sorry, I should've been more clear. The RPI is a gateway to VPN, I have configured the VPN to whitelist lan ip, and have enabled the RPI as a network gateway and it does work when I manually set other devices to use it, currently I'm using that 2nd router to specifically go through the RPI as some devices only supports DHCP, which are connected to the 2nd router.
I'm not at all sure my goal is do-able as it's not a usual dual-wan setup, as part of the subnet serves as a gateway to another subnet, similar to a chained proxy only more complicated I imagine.
Provided that dhcp server here is the OpenWrt, you can use the option3,192.168.1.139 in advanced settings of the lan interface for dhcp and advertise the RPi as default gateway.
Or if you don't want to do it for every host, you can use tags.
I know that advanced settings part which I didn't dare to touch, is that however different from setting the IPV4 gateway in the interface settings? I mean, I put 192.168.1.139 in that interface settings as the 2nd subnet gateway, which didn't work, should I have instead put 10.0.0.1 in there and leave 192.168.1.139 for the advanced settings?
you use policy rules... ( ip route / ip rule ... )
uci set network.route1="route"
uci set network.route1.target="0.0.0.0/0"
uci set network.route1.gateway="192.168.1.139"
uci set network.route1.table="10"
uci set network.rule1="rule"
uci set network.rule1.src="10.0.0.0/24"
uci set network.rule1.lookup="10"
uci commit network
service network restart