My SMS are not displayed in macOS Messages app anymore
For SMS to appear in Messages app in macOS, you need to have Text Message Forwarding turned on in your iPhone. It can be enabled by going to Settings apps → Messages → Text Message Forwarding on your iPhone and turning on the switch which appears against your macOS device in the list.

The iMessage app on iPhone can send both an iMessage message as well as a SMS. iMessage appear in a blue bubble, while regular SMS appear in a green bubble.

Assuming you are signed into both your iPhone 6 and iPad 2nd generation, you can receive and reply to regular SMS (green bubble) from your iPad (as well as other Mac and iOS devices signed in using the same Apple ID), by using a feature called Text Message Forwarding. The option can be found by going to Settings apps → Messages → Text Message Forwarding and turning on the switch which appears against your iPad device in the list.

For step by step instructions, you can go through the Apple knowledge base articles, Use Continuity to connect your Mac, iPhone, iPad, iPod touch, and Apple Watch and How to forward SMS/MMS texts on your iPhone.