I had this same issue with the Poco F3, looks the same because you dont lose signal, but you lose data service and data calls, the quick fix for this is to drop the preferred network type to 3G.
For me to fix it i had to block one LTE Band (B66) that was causing this issue, i had this issue on and off for a while, and it started when i switched from Global to xiaomi.eu ROM.
You already have unlocked bootloader, but you will need root, you need LTE Discovery app to see if it happens when an specific LTE band is being used, after you have identified which one then you need to lock it (block phone from using it), i used Network Signal Guru to do it, i dont remember the exact steps 100% but you just need to use their LTE band lock feature, after i did that im still able to use 4G/4G+ and even 5G bands without any issue.
I took the process from here, hope it works for you, your screenshot looks exactly like what happened to me.
Root is only required to modify the bands, after that you can remove it cause it will cause issues with banking apps (that happened to me), it doesnt affect the modifications after.
c.mi.com
