xiaomi mi 9 miui eu lag issues

[limb0ist]

If I understand you correctly you say that you can not choose different settings for both sim slots? Because for me that does work. My primary sim is set to LTE/WDCMA, and the secondary (empty one atm) is set to GSM only. Works fine . Perhaps you need to disable Dual 4G in: settings --> SIM cards & mobile netoworks. note: I am still in the global rom 10.2.9.0, so I don't know if it's different in the xiaomi.eu rom.
One other question I have is why there is no option for: GSM/WDCMA/LTE. because sometimes GSM is the only available network, especially outside urban areas.

Oh interesting, I couldn't get mine to different signal bands. Well you won't need GSM as if there isn't strong/modern signal band in the places you visit, you will automatically pick up H or H+ signal band instead of LTE. The last two days I was away from the city and can confirm that in places that does not have LTE, it will pick up the on it can.