[SalesForce] All Soap API calls to Marketing Cloud returning 405 error

I have 2 instances of Marketing Cloud I'm currently working with. Both have been configured identically for API access and I created a OAUTH 1 package for Web API integration for each of them.

Security settings for each instance and both users are set the same.

I'm using Postman and the Marketing Cloud library from GitHub to make the calls.

Instance 1 works perfectly. I can make Soap calls to retrieve Data Extensions and create them.

Instance 2 allows me to retrieve a token, but no other SOAP calls work. Every call I make returns the following error:

<h2>405 - HTTP verb used to access this page is not allowed.</h2>
<h3>The page you are looking for cannot be displayed because an invalid method (HTTP verb) was used to attempt access.</h3>

Any idea what could be causing this?

EDIT (2/17/2019): I am using POST and the SOAP Endpoint to make the calls after requesting a token.

Here is an example of one of the Soap calls I made to create a test Data Extension:

    <?xml version="1.0" encoding="UTF-8"?>
<s:Envelope xmlns:s="http://www.w3.org/2003/05/soap-envelope" xmlns:a="http://schemas.xmlsoap.org/ws/2004/08/addressing" xmlns:u="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd">
   <s:Header>
      <a:Action s:mustUnderstand="1">Create</a:Action>
      <a:MessageID>urn:uuid:7e0cca04-57bd-4481-864c-6ea8039d2ea0</a:MessageID>
      <a:ReplyTo>
         <a:Address>http://schemas.xmlsoap.org/ws/2004/08/addressing/role/anonymous</a:Address>
      </a:ReplyTo>
      <a:To s:mustUnderstand="1">{{soapEndPoint}}</a:To>
      <o:Security xmlns:o="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd" s:mustUnderstand="1">
         <o:UsernameToken>
            <o:Username>{{soapUsername}}</o:Username>
            <o:Password><![CDATA[{{soapPassword}}]]></o:Password>
         </o:UsernameToken>
      </o:Security>
   </s:Header>
  <s:Body xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
    <CreateRequest xmlns="http://exacttarget.com/wsdl/partnerAPI">
        <!--
            <Options>
             <SendResponseTo>
          <ResponseType>email</ResponseType>
          <ResponseAddress>your_address@salesforce.com</ResponseAddress>
          <RespondWhen>OnCallComplete</RespondWhen>
          <IncludeResults>true</IncludeResults>
          <IncludeObjects>true</IncludeObjects>
         </SendResponseTo>
             <RequestType>Asynchronous</RequestType>
            </Options>
        -->
         <Objects xsi:type="DataExtension">
            <Client>
                <ID>{{accountId}}</ID>
            </Client>
            <CustomerKey>de_custkey</CustomerKey>
            <Name>DE Test</Name>
            <!-- Specify folder DE created in
            <CategoryID>432655</CategoryID>
            -->
            <IsSendable>true</IsSendable>
            <SendableDataExtensionField>
                <CustomerKey>SubscriberKey</CustomerKey>
                <Name>Subscriber Key</Name>
                <FieldType>Text</FieldType>
            </SendableDataExtensionField>
            <SendableSubscriberField>
                <Name>Subscriber Key</Name>
                <Value/>
            </SendableSubscriberField>
            <Fields>
                <Field>
                    <CustomerKey>SubscriberKey</CustomerKey>
                    <Name>Subscriber Key</Name>
                    <FieldType>Text</FieldType>
                    <MaxLength>50</MaxLength>
                    <IsRequired>true</IsRequired>
                    <IsPrimaryKey>true</IsPrimaryKey>
                </Field>
                <Field>
                    <CustomerKey>EmailAddress</CustomerKey>
                    <Name>Email Address</Name>
                    <FieldType>EmailAddress</FieldType>
                    <IsRequired>true</IsRequired>
                    <IsPrimaryKey>true</IsPrimaryKey>
                </Field>
                <Field>
                    <CustomerKey>FirstName</CustomerKey>
                    <Name>First Name</Name>
                    <FieldType>Text</FieldType>
                    <IsNillable>true</IsNillable>
                    <IsPrimaryKey>false</IsPrimaryKey>
                </Field>
                <Field>
                    <CustomerKey>SignupDate</CustomerKey>
                    <Name>Signup Date</Name>
                    <DataType>Date</DataType>
                    <IsNillable>true</IsNillable>
                    <IsPrimaryKey>false</IsPrimaryKey>
                    <FieldType>Date</FieldType>
                </Field> 
            </Fields>
         </Objects>
    </CreateRequest>
  </s:Body>
</s:Envelope>