Sharepoint Office 365 OAuth Service authentication to list.asmx

I´m trying to access Sharepoint list data in Office 365 from an external website. I registered my app in Azure Active Directory and I've done all the process of creating and trusting a certifacte and getting the access token.

Add-Type -Path ".\Microsoft.IdentityModel.Clients.ActiveDirectory.dll"

$authenticationContext = New-Object Microsoft.IdentityModel.Clients.ActiveDirectory.AuthenticationContext -ArgumentList "https://login.microsoftonline.com/{myTenantId}/", $false

$cer = New-Object System.Security.Cryptography.X509Certificates.X509Certificate2
$cer.Import(".\WithPrivateKey.pfx", "privateKey", [System.Security.Cryptography.X509Certificates.X509KeyStorageFlags]::MachineKeySet)

$clientAssertion = New-Object Microsoft.IdentityModel.Clients.ActiveDirectory.ClientAssertionCertificate -ArgumentList "{myClientId}", $cer

$authenticationResult = $authenticationContext.AcquireToken("https://{tenantName}.sharepoint.com", $clientAssertion)

$headers = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
$headers.Add("Authorization", "Bearer " + $authenticationResult.AccessToken)

I can successfully call to Sharepoint REST Api by presenting the access token in request headers.

$response = Invoke-RestMethod -Uri https://{myTenantName}.sharepoint.com/sites/devSite/_vti_bin/ListData.svc/TestList -Method Get -Headers $headers 

However I can't do the same to access asmx endpoints, such as Lists.asmx, whenever I try to call any method on those services I get 401 UNAUTHORIZED

$body = '<?xml version="1.0" encoding="utf-8"?><soap12:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://www.w3.org/2003/05/soap-envelope"><soap12:Body><GetList xmlns="http://schemas.microsoft.com/sharepoint/soap/"><listName>TestList</listName></GetList></soap12:Body></soap12:Envelope>'

$response = Invoke-WebRequest -Uri https://{myTenantName}.sharepoint.com/sites/site/_vti_bin/Lists.asmx -Method Post -ContentType 'application/soap+xml' -Headers $headers -Body $body