How to organize interaction between internal and external servers?

Question

Yii

Colleagues, hello. Prompt what variants of the organization of interaction between internal (inaccessible from the Internet) and external servers are.
Essence:

• there is a site accessible from the Internet (on Yii 2.0),
  
• there is an API available from the Internet (on Yii 2.0),
  
• there is an internal (inaccessible from the Internet) server (on Yii 1.0) with which the API communicates
  

Now such architecture works on VPN. But this creates problems: the VPN sometimes drops, the speed leaves much to be desired. Are there other options?

Answer 1

Shake the /29 white IP subnet from the provider, deal with routing - achieve stable VPN operation.