Connecting two networks with OpenVPN

I have two routers in two cities, both is connected to the Internet via different ISPs. I wanted to let computers behind both routers to be able to talk to each other directly (without port forwarding and other stuff).

Firstly, I set up a OpenVPN server in p2p mode with static keys, whose IP address is 129.168.20.1, on one router, and a p2p client, whose IP address is 192.168.20.2, on the other router. Below are their configuration files:

p2pserver.conf
-------------------------------
mode p2p
proto udp
port 2333
dev tun
ifconfig 192.168.20.1 192.168.20.2
secret static.key
mssfix 1450
verb 1
log /tmp/p2p.log
keepalive 10 120
persist-tun
persist-key
route 192.168.99.0 255.255.255.0

p2pclient.conf
---------------------------------
mode p2p 
proto udp 
dev tun
remote server's_domain name 2333
ifconfig 192.168.20.2 192.168.20.1
verb 1
secret static.key
keepalive 10 120
log /tmp/p2p.log
mssfix 1450
route 192.168.33.0 255.255.255.0


There is one thing to notice. You should add route of the other end's subnet, so that all traffic to the other subnet will be forwarded through the tun device. To enable forwarding through tun device, you should add firewall rules.

Firstly, add following lines in /etc/config/network

config interface 'p2phome'
option ifname 'tun0'
option _orig_ifname 'tun0'
option _orig_bridge 'false'
option proto 'none'

Then, add the new interface to the lan zone of firewall. And enable forwarding in the lan zone. This can be done with OpenWRT's Web UI.