Brother MFC J4340DW and ASUS RT-AC88U Router. I have had this problem too but none of the advice in here was able to fix it but I have found a solution that now works for me. I have the Brother MFC J4340DW printer and an ASUS RT-AC88U router. You need to log into the Router go to >wireless>professional> and then disable "Airtime fairness" and make sure you do this on both 2.4 and 5GHz bands. This worked for me, my printer no longer goes offline. Also if you are having problems with your Sonos speakers not connecting this will fix that too