I am trying to get L2TP (client-to-LAN)

I have two routers.   1 cable modem #1 -> TPLink for inbound VPN connections

                                 1 cable modem #2 -> router for outbound connections (LAN and VPN) 

                                 The reason for two cable modems is that Xfinity filters inbound L2TP VPNs if the cable modem is not in bridge mode.  Bridge mode breaks accessing Xfinity via website and xfinity applications 

On a Windows 10 PC, everything works as expected.  Inbound in TPLink/Cable modem 1 and out bound on cable modem 2

On iOS (up to date) I can connect to the LAN and Internet.  As expected inbound into modem #1 works.  However, all traffic goes out cable modem 1.  How do I force all internet traffic out cable modem #2

I tried to create a status route 

Destination 0.0.0.0

subnet 0.0.0.0 

Next hop: the LAN IP of router connected to cable modem 2 

But when I go to save it says 

Invalid parameters.