My carrier (Swan) only has cell towers in 1800MHz band. They partnered with other carrier (Orange) to extend their coverage. Originally, this was done using so called “National roaming” in 2G and 3G. For purposes of internet connectivity, 2G is irrelevant. This was awesome as I could just manually choose Orange and get faster speeds. Unfortunately, Orange shut down their 3G network, and the license was updated so they now provide Swan with 4G except in 800MHz band.

What’s different? It’s not done via “National roaming” anymore, but the phone signs into Orange’s network natively as Swan, without roaming, and it is not possible to manually select Orange anymore.
So, how would MediaTek help me?
They have “Engineer mode” *#*#3646633#*#* with “Band mode” selection where you can allow specific bands manually.
Remember that Swan only has towers in 1800MHz band? Yep, I could disallow that, and stick to Orange towers (also limiting myself from their B3 towers, but whatever).

I have tested that with my old MediaTek phone, and it works. So it’s a functional concept.
(Same thing can be achieved on rooted Qualcomm and app like NSG)

I found one more workaround (no, not using a jammer which would be illegal). I found out that I won’t get switched away from Orange as long as there is a continuous connection. So, I can take a bus into area without Swan coverage and connect to a VPN using OpenVPN TCP (didn’t help with UDP), and then head back. Important thing is to never disconnect, not even for a second.
That’s how I am currently on 2100MHz from Orange. I must stay connected 24/7.