IPv6 and routing mobile data

[puffchumpy]

Applications that try IPv6 connections first seem to use mobile data when the mobile data connection wwan0 has a valid IPv6 address and route when the wifi wlan0 only has IPv4. I suspect this is the reason why my November prepaid 500mb data has been consumed. It's my PinePhone test sim, so no loss. 

RedPocket a US MVNO on T-Mobile provides dual stack IPv4 and IPv6 connectivity. My home ISP does not provide IPv6 connectivity. This mismatch seems to be causing problems. My Mobian PinePhone was connected to my home network via usb-ethernet, wifi(wlan0) and mobile data(wwan0) when I apt installed some programs. The download process seems slow and then failed. I didn't check apt's download connections but I suspect they were using IPv6. Consumed the remaining monthly data allowance and then started to fail.

Any thoughts on adjusting the default Mobian routing to prevent applications from using IPv6 wwan0 mobile data when IPv4 WIFI or Ethernet is available? I assume there are some networking setting that can be used.

IPv6 could be disabled, not my preferred solution. T-Mobile and its MVNOs seems to prefer MMS messages get downloaded via IPv6. I've been using Janky-mms and would like to continue to have the ability to download MMS content.

[dukla2000]

My waters have been saying my mobile data usage has been higher than I expected - this would explain it. Need to run some tests to confirm.

[bcnaz]

Not certain if it is still true,  but, Whenever I attempted to download data or updates via data connection, 
     I get warnings to use a different internet connection.

While downloading data or updates through wifi it just works,  no warnings.

[dukla2000]

My waters have been saying my mobile data usage has been higher than I expected - this would explain it. Need to run some tests to confirm.

Quote myself - how sad!

But after tests my PinePhone is using no mobile data as long as WiFi is enabled. I am confused (too dumb?) to work out how my phone is routing mobile data - it is using mobile data as I write this but as far as I can see wwan0: is neither IPv4 nor IPv6!

Code:

$ ip a
...
4: wwan0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1358 qdisc fq_codel state UNKNOWN group default qlen 1000
    link/none
    inet 10.94.204.68/29 brd 10.94.204.71 scope global noprefixroute wwan0
       valid_lft forever preferred_lft forever

This is LTE on Three UK.