Tethering Connection with Boost Mobile Very Unreliable?

I've just start using Boost Mobile not too long ago.
When I first started using it and tested using speedtest.net I got a very high download speed maybe around 60+mbps or so.
However after some time if I go somewhere while needing to work, I tried to tether the data using my laptop and run a remote desktop connection and weirdly found the connection to be so unstable/slow that basically the RDP connection keeps dropping/unusable.
On the other hand if I use a Lebara sim on the same handset (2nd slot), I could actually tether and the RDP connection is usable. Previously I used Kogan mobile and never had problem like this either.
I've tried with multiple places, but I have never been able to get a stable connection to connect via RDP when using boost mobile.

I'm just wondering if there's any restriction placed or is there any setting that could cause the issue or not?
I just found it very weird that if I don't tether the data and test via speedtest.net, the speed is crazy good, but when I start to tether, the connection is so bad that the RDP connection keeps dropping, and I could hardly open any webpage either.
I still have 8 months left on boost, so not being able to tether the data for working purpose is not ideal.

I'm using Xiaomi Mi Note 10 btw.