Trouble with Port Forwarding on DD-WRT

[itsuki2020 0]

I am trying to run services behind AirVPN on ports 3637 and 15744.

The way my network is configured is as followed. I have a modem, which goes into my main router, which is not behind a VPN. A secondary router is connected to my main router, running DD-WRT and is configured to run AirVPN. This seems to be working properly as the devices connected to the secondary router show a public ip belonging to AirVPN. To forward the ports, I ran the following commands.

iptables -I FORWARD -i tun1 -p udp -d 192.168.99.15 --dport 3637 -j ACCEPT
iptables -I FORWARD -i tun1 -p tcp -d 192.168.99.15 --dport 3637 -j ACCEPT
iptables -t nat -I PREROUTING -i tun1 -p tcp --dport 3637 -j DNAT --to-destination 192.168.99.15
iptables -t nat -I PREROUTING -i tun1 -p udp --dport 3637 -j DNAT --to-destination 192.168.99.15
iptables -I FORWARD -i tun1 -p udp -d 192.168.99.15 --dport 15744 -j ACCEPT
iptables -I FORWARD -i tun1 -p tcp -d 192.168.99.15 --dport 15744 -j ACCEPT
iptables -t nat -I PREROUTING -i tun1 -p tcp --dport 15744 -j DNAT --to-destination 192.168.99.15
iptables -t nat -I PREROUTING -i tun1 -p udp --dport 15744 -j DNAT --to-destination 192.168.99.15

However, port checking tools like canyouseeme cannot see my services, and I am unable to accept incoming connections. Any help would be greatly appreciated.