Hi @babak hosseinzadeh ,
Thank you for your response and bringing your concern to us as you mentioned that you are using pay-as-you-go subscription so, as per my knowledge if you want to access the developer portal in APIM we suggest you upgrade the APIM resource to any of the SLA having tier.
As per your concern the above document is correct but you are using pay-as-you-go subscription so the configuration might be varied.
Thank you.