Consulting for home automation solution

I am planning a home automation project and need some consulting by phone and Signal/Whatsapp chat.

Requirements:
- Ample knowledge of open source home automation solutions
- Knowledge of Narrowband IoT (NB-IoT)
- Very good English skills (no strong accents please)

My current set up:
- Several homes, each equipped with one or more smart power switches
(example: https://www.shelly.cloud/en/products/shop/shelly-pro-1pm and/or https://www.shelly.cloud/en/products/shop/1xplug )
- Each power switch is connected to the Internet (Wifi) but is NOT reachable from outside (firewall)

Goal:
- I wish to be able to turn individual power switches on/off from a central server (somewhere on the Internet), driven by a custom script (logic will be supplied by me)
- Central server should be based on Debian Linux, if possible
- Open source software only
- No third-party cloud solution

Questions:
- How to connect those power switches to the central server?
- How to make them poll the server for status changes?
- Do we need to change firmware on power switches?
- Software setup for central server
- Would it by feasible to use NB-IoT instead of Wifi. What are requirements, are there any power switches on the market that could use NB-IoT instead of Wifi?
- In a next step, we might add temperature sensors that report current temperature to the central server. Sensors must by battery-based. Do you have any suggestions for this?

Please show me some examples of your previous projects in this field and quote me with an hourly fee for the consulting.