Hi,
I had a old router in my house and I thought why not I connect the old router to Arduino UNO R3 and on of that I will put a Ethernet shield and join it to my new router
Can I do it and create a WiFi ?
Your topic does not indicate a problem with the cloud editor and therefore has been moved to a more suitable location on the forum.
You can if the UNO has WiFi. The R3 did not but a new R4 does.
Although Wi-Fi is a convenient way to connect an Arduino board to a router, it is not necessarily the only option. Perhaps there are some models that don't, but even Wi-Fi routers typically have Ethernet jacks to which you can connect an Arduino board via an Ethernet cable.
So @robotic_7714's plan to use an Ethernet shield to connect the UNO R3 to the router is certainly viable.
It is possible that the router in question doesn't even provide a Wi-Fi access point. Perhaps those are more rare today, but certainly many models were Ethernet-only in the years gone by, and those models were less expensive than the Wi-Fi models.
I am not sure either of us is understanding his second language. The more I tread it the less I understand it. Maybe draw a picture?