Invoke-RestMethod returns "The underlying connection was closed", URL shows "Connection was reset"

Question

I'm trying to send a body of JSON using PowerShell from several systems to my Flask website.
While it works on most of my systems, but fails on some of my systems and I couldn't figure out why..
PowerShell command:

$res = Invoke-RestMethod -Method 'Put' -Uri $url -Body (ConvertTo-Json $dut -Depth 3) -Header @{"Content-Type"="application/json"};

The error shows:

Invoke-RestMethod : The underlying connection was closed: An unexpected error occurred on a receive.
At line:1 char:8
+ $res = Invoke-RestMethod -Method 'Put' -Uri $url -Body (ConvertTo-Jso ...
+        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-RestMethod], WebException
    + FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeRestMethodCommand

On the systems that work, I could browse the website on them at $url without any issues
But on the systems that fail, the browser could NOT reach the website at $url with the error "The connection was reset", and I think this explains why PowerShell script fails on them also.

I've tried comparing the outputs of [enum]::GetNames([Net.SecurityProtocolType]) on the passing and failing systems, and everything looks the same:

SystemDefault
Ssl3
Tls
Tls11
Tls12
Tls13

I've also looked up many other similar posts and most of them say this command could fix their issue: [System.Net.ServicePointManager]::SecurityProtocol = [System.Net.ServicePointManager]::SecurityProtocol - bOR [System.Net.SecurityProtocolType]::Tls11 -bOR [System.Net.SecurityProtocolType]::Tls12, but it didn't work for me and I also think the output of [enum]::GetNames([Net.SecurityProtocolType]) already proved that my passing/failing systems are both using these protocols..?

I'm not an expert on these protocols and can't find anything so far on how to resolve this.
Could someone please give me some directions?
Thanks!