You miss understand the need for a static IP address. A static IP address is an address that never changes. It is your IP address. A static IP address is not required to get OpenVPN to work. A static IP address does not require a
DDNS. Because you know what the IP address is and it never changes. A static modem MAY be required depending on your internet provider.
Most of us have dynamic IP address, these are IP address that are assigned to connect to your network by your internet provider. This address can change. Normally they change very infrequently, daily, monthly, after a reboot or a power failure. The purpose of a DDNS a dynamic directory name server is to keep track of the address by use of a name. Lets say that your DDNS name on your Asus router is XYZ1234.Asuscomm.com. The router send your current IP address and name to asuscomm, so when you access XYZ1234.Asuscomm.com it returns the current IP address. If the ip address changes then router send the new ip address to asuscomm. So any time you access XYZ1234.Asuscomm.comyu get the IP address.
There are problems with this service depending on who and how your internet provider assigns IP addresses. Some internet provider constantly change the IP address, the only place I have seen this is on cell network that is using a hotspot.
Some intern providers block inbound traffic connection.
1)What type of internet connection do you have, (cell, coax, dsl (wired phone), satellite, fiber,radio beam.......)
2) who is your internet provider ?
3) what are the first 2 numbers of the IP address on the asus router home screen ie 76.31.xxx.xxx
4) what is the first 2 numbers of the IP address on Google "what is my ip address"
5) What is the make and model number of you modem.
6) is your modem only a modem or is it a modem/router ?