I have the same preference mode on my iMac running Mojave and i never had any issues. I always connect to 5Ghz and speeds range b/w 95-110Mbps on downloads. On 2.4Ghz the speeds range between 75-50Mbps depending upon the proximity range.this laptop thing is very weird.
5Ghz network on laptop 8 Mbps on speedtest
5Ghz netwrok on mobile 95+ Mbps on speedtest.
Edit: I think I found a solution to this issue. This all started when I switched the order of preference of joining wifi networks on my MacBook Air. I had moved 5 GHz band over 2.4 GHz band preferring 5 GHz and the speed fell. Now I have switched it back i.e. 2.4 GHz will get preference, it is back to full speeds on 5 GHz band. WEIRD.
There are actually 6 Access Points (3 for 2.4GHz, 3 for 5GHz).Is there any way to disable just the 2.4 GHz band on Jio ONT and use Mi 3C router instead? I have a theory that a dedicated router would provide better speeds.