Rest API and Powershell

I'm working on transitioning some of our custom reporting over from SCOM to LM as we transition.  I'm having a difficult time getting REST to give me anything at all.  I have an API Token created and have saved it into my credential store.  I'm using the CredentialManager 2.0 module to access them.  Here's the rest:

Import-Module    CredentialManager
$Cred           = Get-StoredCredential -Target LogicMonitor

$accessId       = $cred.UserName
$accessKey      = $cred.GetNetworkCredential().Password

$company        = "ourcompany"

$httpVerb       = "GET"
$resourcePath   = "/device/devices"

$epoch          = [Math]::Round(
    ( New-TimeSpan `
        -start ( Get-Date -Date "1/1/1970"  ) `
        -end   ( Get-Date ).ToUniversalTime() `
    ).TotalMilliseconds
)

<# Concatenate Request Details #>
$requestVars    = $httpVerb + $epoch + $resourcePath

<# Construct Signature #>
$hmac           = New-Object System.Security.Cryptography.HMACSHA256
$hmac.Key       = [Text.Encoding]::UTF8.GetBytes( $accessKey )

$signatureBytes = $hmac.ComputeHash( [Text.Encoding]::UTF8.GetBytes( $requestVars ) )
$signatureHex   = [System.BitConverter]::ToString( $signatureBytes ) -replace '-'
$signature      = [System.Convert]::ToBase64String( [System.Text.Encoding]::UTF8.GetBytes( $signatureHex.ToLower() ) )

<# Construct Headers #>
$auth           = "LMv1 $accessId`:$signature`:$epoch"
$headers        = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
$headers.Add(   "Authorization",$auth              )
$headers.Add(   "Content-Type" ,'application/json' )

<# Construct URL #>
$url            = "https://$company.logicmonitor.com/santaba/rest$resourcePath"

<# Make Request #>
$response       = Invoke-RestMethod -Uri $url -Method $httpVerb -Header $headers

I'm getting this response:

Invoke-RestMethod : The underlying connection was closed: An unexpected error occurred on a send.
At line:1 char:19
+ ... nse       = Invoke-RestMethod -Uri $url -Method $httpVerb -Header $he ...
+                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (System.Net.HttpWebRequest:HttpWebRequest) [Invoke-RestMethod], WebException
    + FullyQualifiedErrorId : WebCmdletWebResponseException,Microsoft.PowerShell.Commands.InvokeRestMethodCommand

what am I missing here?  All of the parts look right going into the invoke:

PS C:\Users\cole.mcdonald> $url;$httpVerb;$headers
https://beyondimpact.logicmonitor.com/santaba/rest/device/devices
GET
Key           Value
---           -----
Authorization LMv1 m27K656aHIjt946ST92q:ZGMzNjk5NTIyZDE2YzMyYTBiOTc0MTFiM2U4MGVmOGQyMWNjNGRiOWNmZDJjNjU2Y2VjYjQxNWViOTg2MDI0NQ==:1553558842750
Content-Type  application/json

(Moderator: please feel free to move this reply if necessary - Not sure if you want this in a new thread or not.)