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What RN7 model variant do you have? Is it M1901F7H or M1901F7G? The G variant is slower since it doesn't have LTE Band 28. Aside from that, your internet connection speed depends on various factors such as the time you're using the internet, proximity to the cell tower and the number of users connected to that tower and using it at the same time and finally your location if your mobile provider is available in your area.
src : David J
Since you mentioned you updated your OS, I presume that you're on MIUI version 10.3.6. and the problem started after updating. Have you factory reset your phone? Since it might just need a clean install to get rid of bugs you're currently experiencing. I know it's troublesome to factory reset as it's such a hassle to set up the phone from the scratch, but we need to know if the problem will go away after doing so.
To fix it, we need to report bugs/glitches that your device is currently experiencing to Xiaomi Developers so that they can issue an update that will resolve your problem. Kindly provide the necessary details such as MIUI version, Model name (RN7 variant), and describe what, when, where and how did the problem occur. You can find your device details at Settings: About Phone and for reporting the issue Settings: Feedback.
Just to clarify: Did you tried calling your Internet Service Provider to check if they're doing any maintenance around your area? Did you also checked if your SIM is already data-capped or not? So far, your thread does not indicate any speed tests with regards to your internet, please show speedtests.