Custom DDNS(having ISP DDNS) and limited ports on UPNP questions

[Wutikorn]

I have to use my ISP DDNS as I have CGNAT, I think that is what RMerlin calls it. Since AsusWRT-Merlin supports custom DDNS, is there a way not to do DDNS script? This is because the router does not have to send new IPs to my ISP DDNS server when my IP changes as they already know. I just want to add DDNS name because when I export my OpenVPN client, and do stuff with AiCloud/Asus Router App, it will know what my DDNS is. However, if I don't do DDNS-start script, there will be error message. So is there an easy script that I, without much knowledge, can write to make router knows that it does not need to start any service, yet creates no error message?

Since I have CGNAT, I only have 10 external ports that I can use. Six are occupied by other stuffs, and there are only four left. Therefore, I have set UPNP: Allowed external port range to those four ports. Will there be a problem if many devices want to use UPNP, but there are not enough ports?

 

[AlexHG]

Sorry this is an old message, but I just joined the forum.

I have both the ISPs in your footer, for AIS I called them and I pay an additional monthly fee of 200 baht to be allocated an ipv4 public IP.
3bb already included in the package, but i needed to phone them to say that i was using a home cam system and please let me have open access.