Slow WiFi speed, compared to other models

TP-LINK AC1200 Archer C50 V3 .

802.11ac Wi-Fi standard is supported at 5GHz. At 2.4GHz, it's 802.11n/g/b

But still, it is up to 300Mbit/s at 2.4GHz. Also it is OK in other devices, laptops, other iPhones (or Android phones).

I'm afraid of having issues in device itself, because it is new((

I've tried WiFi Sweetspot app, from AppStore to check speed between my router and phone. My router gives 20-30Mbits/s. Then checked in two different routers and got enough 70-80Mbits/s. (All is done in 2.4GHz). Checked network configurations on all devices (that all Tp-Link brand), all pretty same, but just different models. Now question, HOW is that possible? ))))

New information. I have checked the speed on 3 wifi routers (2 TP-Link & Tenta), using WiFi Sweetspots app. My phone gives unstable records, from 10 to 70Mbit/s (jumps & falls continuously). Tried other phones (of my colleagues, iPhone11ProMax, GalaxyA50, XiaomiMi9T) all of them got stable 80-90Mbit/s. It seems to me that it is because of my iPhone XR (((((

I've enabled 5GHz also. there result is around 30-40 but still not around 100 (I do local speedtest to my ISP servers which almost always gives 100Mbit/s).

I've updated it, didn't work. I need to mention that I have bought it from South Korea in 2017. Maybe it does have some sense

And that's the problem, that there is no an Authorized Apple Service Provider in my country (Uzbekistan) (((