How exactly can I mount an external hard drive on my home Deco M5 Mesh network? I am also open to any suggestions that achieves my desired functionality of a file-sharing system on my local network, preferably with backup/redundancy.

I have done my homework, and most of the solutions that I have found are not suitable for the following reasons:

1. This solution: https://community.microcenter.com/discussion/3956/connecting-external-hard-drive-to-wireless-network says that the external drive can be connected to the Deco device via a "NAS adaptor", but I cannot find out precisley what a "NAS adaptor" is or where I can purchase one. I did not see anyone on Amazon.com.

2. This solution https://www.reddit.com/r/HomeNetworking/comments/hqotyt/connecting_external_hard_drive_to_tplink_deco_m5/ suggests building your own NAS device (!!). I am not interested in building my own NAS device. 

Note: most NAS systems out there appear hard to set up, very expensive, or are not secure (i.e. are not just local to the network, and allow general internet access). I am theoretically open to NAS systems, but I would need a specific recommendation of what type of NAS system with a cost of less than $300.

Other relevant sources I looked at:

https://community.tp-link.com/us/home/forum/topic/166176?replyId=342262

https://www.howtogeek.com/190835/how-to-turn-an-old-pc-into-a-home-file-server/