I can't connect some multiplayer games even though I have internet connection

I just called my internet provider and they tell me to call the modem manufacturer.

and before, when I called my modem manufacturer for other issues, I had to do all the things they said myself because they did not have direct access to the modem.

You can say something like this to them:

"I need to open some ports in my router, these are the ports:

*Then you can read the numbers and the protocol (UDP, TCP)*"

They know what is UDP and TCP (Both are protocols)

So, just say to them the port number and next the required protocol (or protocols for each one)

I get it, well, one last thing. In my speech, do I have to say something like I will request to open a port for them to understand? .d

If you check again your NAT type. is appearing the same message!

Unavailable and can't get teredo IP?

Could be an strange coincidence, but probably after a certain time you are going to face the same issue

Sorry to say but your ISP gives you wrong information.

They're most than capable to do this for you.
The manufacturer can't assist you due to they don't have remote access to your router, they can share you guides on how you can do it manually.

And you need a PC to do this manually, I can share you a detailed tutorial. But as you don't have PC you need to give them a new call and ask them one more time. Maybe the person who you talked didn't know how to do it.

But they can do it.

Tutorial (with PC) : https://portforward.com/how-to-port-forward/

** This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.