Unknown devices connecting to home network

I have recently installed xfi app and to my surprise I saw a lot of devices connected to my gateway. Apart from my own tv, chrome cast device, iPhone and samsung phone there were over 20 devices connected. These unknown devices includes-
1. Multiple IoT devices
2. Expressi devices
3. AmazonTE
4. Amazon firetv
5. Amazon echo series speaker
6. Technico device
7.Many ESP devices
8. Wyzecam camera
9. Multiple Ipad
10. Apple TV
11. Iphones

I have taken below steps already:
1. Updated wifi password
2. UpdateD both wifi name and password
3. Factory reset setting of router.
4. Hidden the network

No matter what I do, these devices keep on connecting to my router/gateway.

I am seriously concerned about security