Which arris modem as there are loads of them.. and there is major issues between SB series,, like SB6141 which is extremely popular and the airport series.. Nothing wrong with either of them but they do not play well together for whatever reason.
If you have ethernet use it.. setup on ethernet is reliable.. setup on wireless can never be reliable.
If you have an ipad or iphone use the iOS version airport utility which is more reliable than Mac versions.
Or in the end if you have no choice.. factory reset the TC.. no files are lost doing this.
Power off the TC.. hold in reset and power on again.. keeping reset in the whole time and still holding in for around 10sec until front LED flashes rapidly. It will then show up in the airport utility.