Your question/your setup is a little confused.
I assume that you have the iptables rule on your server. You want to connect from the internet to your server using UDP port 2000. The openvpn server listens on UDP port 4253.
You need to get sure your router is correctly configured: You need to redirect external UDP port 2000 to internal UDP port 4253 on your server.
You need also to modify your iptables rule: It should be for UDP, not for TCP. Additionally, your server gets the UDP traffic to port 4253 as your router rewrites the destination port and IP:
iptables -t filter -A INPUT -p udp --dport 4253 -j ACCEPT