Hello to everyone! 
First of all, I've used the search button and I've come across this thread for example. I've also taken a thorough look at the Wiki but I couldn't solve it by myself and that's why I'm asking for help here.
At home I get both data and voice from the cellular service and I use a Huawei B310s to achieve this. By what I know, nothing similar is obtainable with an OpenWrt box alone given that a 4G USB stick like the E3372 doesn't handle the voice service and, even if it did, then I don't know how to route it to the analogue telephones at home. The B310s misses all the rest though so I use also an OpenWrt box which gets internet connection from it and does all the rest I need.
I've put them on two different subnets like, at least by what I've understood, it is adviced to do under this kind of scenario. The Huawei is on 192.168.8.x while OpenWrt is on 192.168.1.x and the connection between them is through WiFi, I have no choice.
I have a desktop which can't be near the Huawei so I keep it hooked to the OpenWrt box that also acts as a repeater BUT I need to remote control that desktop so I need to open ports for VNC in the firewall. I've done it on the OpenWrt box but it isn't enough (I already expected that, not a surprise) and I can't open ports for that IP on the Huawei because it is on a different subnet.
What can I do to get VNC to work? I know about OpenVPN etc. but I'd like to start from the simplest approach and add refinements later on. Thanks!
So if I understand the question here, you are trying to enable VNC with port forwarding from the internet to your PC, where your topology looks like this:
Internet > Huawei Router > OpenWrt > PC
Your PC is behind double-NAT... so you need to open ports twice. First, you need to open ports on the Huawei router pointing to the WAN IP of the OpenWrt router (which will be on the 192.168.8./024 network). Then, on the OpenWrt router, you'll open the ports pointing to the PC on the 192.168.1.0/24 network.
First of all, thanks for your follow-up! You've understood just fine and what you say makes absolutely sense. I would apply the configuration ASAP and mark the thread as solved after doing it. Have a nice day!