Need help with "Global" ROM

TL;DR

Bought a flashed "Global" Xiaomi 15 after seller/store told me it has e-sim only to find out that it has no e-sim and cannot be updated anymore to new versions of the os.

For context, I am not an expert talaga of this field, just someone who ended up reading a lot of forums and posts after a failed expectation. So here's the story, after spending months thinking and contemplating, I finally decided to buy a new phone. I decided on a CN ROM since it's bang for the budget, as per most reviews. Then I stumbled upon a seller/store that has been supported by a local YouTuber that has the unit that I slowly narrowed down. When I asked for a CN ROM, I was offered a flashed "Global" ROM variant of the model I chose. Again, I am not an expert. When I asked and was told that the phone has e-sim, I immediately jumped the gun (a terrible and regretful oversight on my end). A month has passed and I only got to use the e-sim feature recently for an international trip only to have the phone reject the qr code of the e-sim I bought (buti nalang na-refund pa yung e-sim). Only then did I know na the e-sim feature pala is just a remnant after flashing the phone with a "Global" ROM. I'll keep saying this, I did not know better talaga since I trusted the seller/store recommended by a local techtuber. Aside from the e-sim feature not functioning, I also can't update the phone. I always thought na delayed lang Yung update for my model only to find out na a year has passed Pala since an updated version has been rolled out. Yung "Global" ROM pala is just an English version of the CN ROM na blocked Ng OTA updates since technically not formally supported. I need help, how can I update my phone? It is a Xiaomi 15 (non Pro). Sabi sa iba, I can apply to unlock bootloader daw but another info Naman states na I should have it returned to an official CN ROM. Di ko alam ano Yung gawin ko. I spent around 25k for the phone when my initial options were the OnePlus 6 Ace, GT7 Pro, Turbo, and a CN Xiaomi 15.