Hi I am having the same issue and have done everything you have recommended and including looking at Wireshark captures but am also still getting 404 errors! Postman seems to be using TLS v1.2 but I have played with the settings to see if it makes a difference and it doesn't.
I simply want to create an SSID via POST in postman... as follows:-
https://n194.meraki.com/api/v0/networks/L_672162244385048740/ssids/0
I have tried without the shard using https://dashboard.meraki.com and https://api.merkai.com aswell and am asusing these both should in theory work as they will just reditrect the https request ? But neither works.
The JSON body part is as follows:-
{
"number":0,
"name":"TEST",
"enabled":true,
"authMode":"psk",
"encryptionMode":"wpa",
"psk":"abcd1234",
"splashPage":"Click-through splash page",
"perClientBandwidthLimitUp":0,
"perClientBandwidthLimitDown":0,
"ssidAdminAccessible":false,
"ipAssignmentMode":"Bridge mode",
"walledGardenEnabled":true,
"walledGardenRanges":"192.168.1.1/24 192.168.37.10/32"
}