Setting static IP on BBB with Debian 9.3

[not you]

I follow this guide to set static IP for my BBB with Debian 9.3: http://derekmolloy.ie/set-ip-address-to-be-static-on-the-beaglebone-black/

My /etc/network/interfaces file is modified to be as follow:

---------------------------------------------------------------------------------------------

# interfaces(5) file used by ifup(8) and ifdown(8)

# Include files from /etc/network/interfaces.d:

source-directory /etc/network/interfaces.d

auto lo

iface lo inet loopback

# Primary / Ethernet port

auto eth0

iface eth0 inet dhcp

address 192.168.0.20

netmask 255.255.255.0

gateway 192.168.0.1

# Secondary / Wifi RT5370

iface wlxe84e0650ff53 inet static

address 192.168.0.21

netmask 255.255.255.0

network 192.168.0.0

gateway 192.168.0.1

---------------------------------------------------------------------------------------------

After that, my ethernet IP is not set to ...20, but ...18 as before.  On the wifi side, I did not put usb0 because it shows up as wlxe84e0650ff53 when I use ifconfig.  Anybody know why?  The extra settings completely kills it.  I got no wifi, unless I comment out these new lines.  Can someone tell me what went wrong?

I did read this post: https://groups.google.com/forum/#!searchin/beagleboard/fixed$20ip|sort:date/beagleboard/3bea2o4iD2s/pFz_VNrvDAAJ

However, I do not know if "[my] router can assign a fixed address or lock the one it assigns" or how to check for that info.  Right now, the username and password for it are not known either.  

Thank you so much.