How to connect to a computer from outside using the RDP protocol? Or how to open a port?

Question

Computer networks

Somewhere 2 years ago it was necessary to urgently connect to a computer via RDP. I read, looked for how to do it, but there was nothing under my router, which the provider sucked in. I didn’t want to figure out the settings of the router myself, I called a person from the provider’s technical support.
As expected, a not very smart person came, who not only had to be told what kind of protocol it was, but he also did not know why and what it was - port forwarding. He called the most important of them in front of me and gave me the phone. I explained everything to him, to which he replied that it was impossible to do this in this router model. I didn’t argue with him that this couldn’t be on modern router models, because I wasn’t sure.
And of course, after two sleepless nights, I did everything myself, opened the right port and connected via rdp. Everything was fine, except that the connection occurred only if the IP was between 90.xx.xx.xx and 99.xx.xx.xx. It didn't bother me too much, so I was happy with it.
I lived for 2 years without problems and used a remote computer almost every day, but now, a couple of weeks ago, I was no longer assigned an IP from the desired range. IPs began to be assigned, which start with 79, 82, 89, 102, 109. I reconnected for a very long time and for more than one day. All the same the necessary ip was not.
How to be? Is it possible to require the desired IP from the desired range in the router settings? Or communicate with the provider again and resolve the issue at their level. They will 100 percent insist on buying a white, static IP.
Router model sagemcom [email protected] 1744, v4.
I tried to prescribe a computer in dmz. Firewalls and other things are disabled and all the necessary settings on the computer are allowed, because before everything worked and it's 100% in ip.
Ps Do not offer all kinds of teamweavers and anidesks as an analogue.

Answer 1

In the settings of the router, you cannot specify that it requires the IP you need from the desired range. If you want a specific IP or from a specific range, then the issue is resolved only through the provider.

Answer 2

the connection happened only if the IP was within 90.xx.xx.xx to 99.xx.xx.xx... IPs that start with 79, 82, 89, 102, 109 began to be assigned

Most likely at the provider new addresses (79, 82, 89...) are covered by NAT'om.
Do one of three things:
1) ask for Provo to open NAT for new addresses;
2) buy statics;
3) buy a dedicated server or virtual machine ( https://poiskvps.ru/ prices from 70 rubles/month) and forward port 3389 from it via VPN (for example, OpenVPN) to your machine.
I would start with point 1. And if they refused, then I would choose from 2/3, depending on the price and my needs (your own dedik can come in handy in other cases).
UPD. By the way, if you have access to a server (for example, at work or with friends), then you can do port forwarding through it - also through OpenVPN or ssh.