If you type whatismyIP.com in your browser, you will see your service provider’s address to your modem.  It is the IPv4 and the IPv6 numbers. If you know the address and port number you can use them to get to your port forwarded device. The issue is that your service provider may change that address. The DDNS is a service that maintains your service providers address to your modem.

 

When you connect to the router, an IP address is assigned to the device such as 192.168.0.119. Every device has a MAC address which most times is printed somewhere on the device. What you want is to use the same device IP address for your device every time you sign on so it can use an assigned to a port. 
Device IP addresses are assigned by the router. If you sign on to your router's basic page, and click on the wireless it should give you a list of the assigned IP address and the MAC address for each connected device. 

The first part of the IP address is the same as your router (most are 192.168.0 or 192.168.1). The last three digits identify each connected device. Some routers have a control range for connected devices. Select an unused higher address. I use 119 through 129. Add an entry in the reservation address table using the MAC address and the selected IP address. 
Restart your router and your device and it should connect using the new selected IP address. 
Now you have an IP address that you can assign to a port on the Virtual Server page. For my cameras I use port numbers 81-89. Not sure what port you need for your device. 
I generally backup the router prior to making changes and restart the router after each change. When things are working, backup again. 

At this point you should be able to access the ported device using the IPv4 address, a “:” followed by the port number something like 55.55.55.555:81.

Since the IPv4 address can change, you register it with a DDNS service. When you register it, you will create a domain name. The domain name will be something like foscam5555.ddns.net if you use NO-IP. You use that name instead of the IPv4 address to access the port forwarded device.

I searched and found there are many DDNS service providers. Also some of them are free to use.
Before using either of them, I have confusions. Is it safe to use free DDNS service? If yes, why they’re free?

And can I use more than DDNS service providers?

You can never be sure that any site especially a free one to be safe. I have used three different DDNS services and not encountered any issues. I tend to use ones that are used by many and/or recommenced by a vendor. I have used NO-IP, DVRLISTS, and foscam. For example, NO-IP has both a free service and a paid service. The free one needs to be verified like once a month or so. You can use more than one DDNS. My router allows only one that will it update with IPv4 changes which I do not currently use since my address has been static. (no need for the router to be talking with some other site). Which means I will need to update each if my service provider changes my IPv4. If you use a VPN service, be sure to use the IPv4 address from your service provider and not one from a VPN service when setting up the DDNS.