Anyone knows why my IP could be being detected as a residential proxy?

Yeah, that's CGNAT alright. My condolences. Now that I think about it, it's even possible that your IP is being seen as a residential proxy precisely because of CGNAT, because it kind of is one in a way (but you have no control over it, so it's pretty unfair on you). I can't say how lucky you'll be in trying to get out; it depends a lot on stuff like your ISP's policies, the terms of your contract, the applicable laws, and so on.

I'm gonna try and explain how CGNAT works in a relatively simple way; it's a bit long, but bear with me. IP version 4 addresses (the ones in the A.B.C.D format, where each letter is a number from 0 to 255) have run out (a long time ago, in fact), and we have far too many devices connected to the internet, so we need tricks to have more devices hooked up than the amount of IPs. NAT (Network Address Translation) is one such trick. Every device on my local network (e.g. my phone, my laptop, my desktop) is behind a NAT; that makes them share the same external IP, so to the internet as a whole they're one machine, and the router does some magic that allows them to talk to the internet and listen back. But if an outside machine tries to talk to them first (as in, start a conversation with my machines, rather than answer when spoken to), my router has no idea what do with that message (what machine should take it?) and just throws it away, which is the big downside. CGNAT (Carrier-Grade NAT) took that idea and applied it for an entire ISP (a "carrier", thus the name), or for a chunk of its users. So you have any number of homes and businesses on the same external IP. If every home has three machines and you have 100 homes behind a single IP on CGNAT, that's 300 machines. All behind the same IP. So it's little wonder that CGNAT looks to the world like a proxy (consider: a proxy is basically something that allows its users, said to be "behind" it, to connect to other machines as if the proxy's IP was their own; that IP, known as the endpoint, hides their real one: you connect from A to proxy and the proxy connects to B, but B only sees the proxy, which forwards the answers to A). Normally, if you don't need to be connectable (a typical use case of requiring that is having security cameras you need to access from the outside), CGNAT isn't a bother, but then you get the situation you're currently facing and all goes to hell. I'm not sure this explains a lot, or particularly well, but I hope it at least helps you see the general picture.

By the way, the long-term solution to this is called IP version 6, but we've been 20 years away from having IPv6 replace IPv4 for the last 20 years, and I think we'll still be 20 years away from it 20 years from now. It's going slower than I could hope to tell you.

That’s unfortunate, but your ISP may be able to provide a dedicated static public IPv4 address. In some cases this is free, though more often it comes with an additional fee. A static IP bypasses the limitations of CGNAT and gives you a direct, consistent connection, which usually resolves these kinds of issues. Keep in mind, though, that not every ISP offers static IPv4 anymore. Some may instead suggest IPv6, which is the future-proof solution, but not all services fully support it yet.