yeah, maybe your got wrong ip address. here is how to fix them:
first: you download this tool: https://shop.bitmain.com/support.htm?pid=00720160906053730999PVD2K0vz0693
then you connect your computer and your l3+ on the same network.
run this tool above and click "Start". also push "Report IP" button on your 3+. you will receive an ip address.
if you dont receive any ip address, try "Reset" button, and then reboot your l3+ and reScan ip again. you will receive your l3+ IP.
go to this ip on your browser with user: root, pass: root
finally change your pool and user!
good luck!!!