eSIM or Dual Line configuration

Maybe to geeky for most:

My iPhone (12 Pro Max 512GB) is set up with a physical SIM card from my local provider (Videotron in Canada), and an eSIM from a french provider (free.fr). Both are fine and run well in general (voice and data flow on both without problems).

Both SIM and eSIM are active, voice dials through the local provider, data flows via the local provider with “Allow Cellular Data Switching” set to “On”.

I ran some performance tests with Speedtest to be sure:

A) Local SIM active, eSIM disabled: Very good download speed, abysmal upload speed.

B) Local SIM disabled, eSIM active: Meh download, decent upload

When I run the test with both SIM and eSIM active, data switching on, SIM as primary, I get the same result as A).

How do I get the iPhone to recognise that it has better incoming bandwidth on the SIM and better outgoing bandwidth on the eSIM, and pass the data back and forth optimally? In other words, how do I get the phone to send packets on whichever SIM happens to have capacity, and sort / reattribute incoming packets properly?

I am thinking here of the tethering use case, where I may be sending emails with large attachments, while my wife is streaming Netflix (or vice versa). As it stands, either she has a good connection and I wait for hours for my mail to go through, or I am happy and she gets choppy video.

I have done this successfully at the office with a Dual WAN Linksys Router. How can I duplicate this on an iPhone?

Please advise.

Best regards, AC