Router 802.11 b is gone. Need for my Roku to connect!

This Roku has been connecting to our Xfinity for years!  Suddenly it's not.  I've tried all the fixes from the Roku end (reconnect, restart, factory reset).  On Roku forums I'm told my router's configuration needs to be b/g/n.  Using the Xfinity app, I found that our router (under 2.4 GHz) only has these options: 802.11g,n or 802.11n. Roku forums say an Xfinity update eliminated the "b" of the b/g/n.  How can I get it back?     Edited to add:  my router is the tg3482g.