I updated existing transformer to 30VA one (see amazon link)
https://www.amazon.com/gp/product/B07GND8RMQ/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1
also updated mechanical doorbell to the one in the ring compatible list (I bought Hamptom Bay’s one)
This is because to prove nothing wrong my configuration or wiring, but it resulted same, the device went to the offline.
Today, finally, I was able to speak to the tech support guy, and accoring to him my Ring doorbell had failed (actually rjected) to update the new firmware. And then it went back to the setup mode after the certain amount of time. I don’t know what actually wrong in my end, but I strongly feel that there’s a problem in the device itself. I will keep trying to discuss with them.