What model is the tablet - Does it have mobile data capabilities?

Have a look for something called "Samsung Secure wifi,"

Its a built in VPN Samsung have been shipping on some devices, and from what I can tell you can choose which apps use it and when... the newly installed apps may be trying to use it 

Once you use more than 250mb a month it becomes a sign up subscription service, so she may have triggered that limit...and without a subscription access is  being denied.  

Yes - I just re-read the whole post and a phone got mentioned as well as the tablet part way down. So an android tablet and a phone have apps that wont work on that wifi....

Seems likely it's something in the router side of things doesnt it?

Maybe a firewall setting in the router - Make/model of router might be useful then.

EDIT: I see slingshot have a 'family filter' facility. 

https://help.slingshot.co.nz/hc/en-us/articles/360000047893-Slingshot-Family-Filter

Shouldn't be the culprit - but is this enabled by any chance?

robjg63:

 

Maybe a firewall setting in the router - Make/model of router might be useful then.

 

From the label:

VDSL/adsl
wifi gigabit modem router
NF4V

Bung: So far you've talked about the problem in terms of new apps loaded onto the phone and tablet. Does anything already on the Samsung devices work on wifi? Do the Samsung devices even see the wifi?

She knows YouTube and the Google app came preloaded on both the phone and tablet, and those do work on her home wifi.

The devices do see her wifi, we "forgot" the wifi this morn and reconnected but it didn't help.

Some older firmware builds for the NF4V did have LAN DNS resolver issues, there have been forum discussions quite a few years back although I have not seen any posts about it in recent years. So if you note the Device Info section perhaps someone here can tell you how to get the latest build for the Slingshot/Orcon modems.

To log into the modem, enter 192.168.20.1 into your web browser address bar.
Username: admin, password: admin (or the last 6 digits of the modem's MAC address which can be found on the underside of the modem).

Some apps may bypass LAN DNS and use DNS over HTTPS (DoH) so that could explain why some apps work and others that rely on your modems DNS don't.

Another possible cause is broken IPv6 connectivity. A simple way to diagnose might just be to turn IPv6 off on the tablet to test. If you cannot find a way to do this, you can also turn it off at the router level by editing the WAN entry in the NF4V to uncheck Enable IPv6. From the hompage / Device Info page you should see whether IPv6 connectivity is enabled as there will be values for Default IPv6 Gateway, Primary IPv6 DNS Server etc..

FINALLY got this fixed, although what the exact problem was remains a mystery.

Slingshot replaced the modem and pinko presto everything instantly started working perfectly.  So something was up with the router.  Possibly the IPv6 thing a commenter mentioned, but not sure.