Using mobile (cellular) data for internet when WiFi has none

[Paddy84]

My previous phone (Moto G5s) would continue using mobile (cellular) data when the current WiFi network had no internet access, whereas my Sony XZ1 won't.  Is there any way I can force the phone to continue using mobile data for the internet without having to turn off the WiFi?

This may seem an odd request, but in my car I have two non-internet-connected WiFi devices that are operated using my phone:  
One is a dash-cam whose WiFi connection is used to stream the video clips and updates its settings to the phone.  Connecting to this then disables my internet connectivity.  If I wanted to stream the video from the dash-cam to my phone while simultaneously using google maps to navigate, I could do this on my Moto G5s, but I can't on my Sony as there's no internet connection.
The other is an OBD2 (On-board-diagnostics) dongle.  This gives me information about how the car is working and is particularly useful in my Nissan Leaf for keeping an eye on energy consumption.  Again, if I want to use this at the same time as navigating, I'm unable to get the route updates from google because my use of the WiFi has caused the phone to lose it's internet connection.

I've had a poke around in the developer options and couldn't find a setting that would seem to do what I want.  Which is, if the WiFi link doesn't have internet access keep it open for apps that are using it specific device while simultaneously keeping the mobile (cellular) data service available for any other apps that need it.
Just to reiterate, I know this is possible on an Android device as it was the default configuration for my previous Motorola G5S.  I just want to know how to get this working on my Sony XZ1.

[NunoSEN2]

Hi @Paddy84 ;

Have you tried going to Settings>System>Advanced>Develloper options>Cellular data always active.

[Paddy84]

Yes.  In the UK at least it's referred to as "Mobile data" and in my case it was active already.  I tried switching on "Aggressive Wi-Fi to mobile handover" as well but it made no difference. 

In any case, I don't actually want it to be handed over, as such.  I want both operating in parallel whenever there's no internet connectivity on the Wi-Fi side.

[NunoSEN2]

@Paddy84 ;

What about the Auto Network Switch in Settings>Network and internet>Wi-fi>WiFi Preferences>Advanced?

[Paddy84]

That is also on, but it doesn't seem to have any effect in my case.

[NunoSEN2 · Accepted solution]

Well @Paddy84, in my case it works, whenever the WiFi network internet fails (with the x over the WiFi symbol) it turns to the mobile data.

[Paddy84]

After a reboot (while still on Android 😎 it seemed to work.  Shortly after that I also got the update to Android 9.0 which I've just applied.  It seems to be working the way I expected now.  Thanks for all your suggestions.