Why does the Sub need to be factory reset? You should be able to temporarily connect it to your router with an Ethernet cable, get the new data about your network to it, and have it bonded to whichever ‘Sonos room’ it was before. There are instructions in the new router - installation instructions FAQ.

If you feel you must reset it, the instructions are in the factory reset FAQ. 

Thanks . I will try the Ethernet cable.  I “reset “ it because that was what the Sonos app suggested I don’t the item could not be found.  Problem is it is supposed to flash green after the reset but it is not happening.

Really sounds like a connection issue.

https://support.sonos.com/en-us/article/sonos-led-lights

I was able to get it to flash green using Ethernet cable . However the system would not “find it” and I could only view it if I set up new system . I am ready to through it out the window

Is the Ethernet on the same LAN as the other Sonos? 

If they are on the same one you should be able to use the Add option in the controller to hookit to your desired Room.

When I try to add it to living room it says it can not be found.  If I try and set up new system, it can find it.

Hi ​@Ben in Taiwan 

Welcome to the Sonos Community!

Sorry to hear of the issue you’ve been having - it sounds annoying!

Some units will need to be connected with an ethernet cable before they will reset, but this is a one-off thing - that Sub won’t need to be connected to ethernet to be reset again.

Please note that a Sub must be added to a system as a new product before it can be assigned to work in a particular Sonos room.

Looking at your system now, it looks like you have now got your Sub working as intended - please let me know if this is not the case and I’ll be happy to help.