No PAYG number or PAN number

I’m a bit confused by your post.

Are your new smart meters configured as PAYG meters?
Or are they credit (pay monthly) meters?

It sound from you post that you may be trying to use the wrong app and the wrong portal.

There are two different apps depending on how you pay for your energy.

One app, the OVO energy Top-Up app, is for topping up PAYG meters, links to download the Androaid and iOS versions can be found here:
https://www.ovoenergy.com/help/article/topping-up-smart-payg-meter

The other app,  OVO energy app, is for credit (pay monthly) customers to view their accounts, download links can be found here:
 https://www.ovoenergy.com/ovo-app

If you are paying monthly rather than topping up a PAYG meter then you don’t need the PAYG OVO energy Top-Up app to top up the meter.
Instead you want the OVO energy app.

Simailarly there are different portals.
For PAYG customers it’s:  https://portal.payg.ovo.com/
For Pay-Monthly customers it’s: https://my.ovoenergy.com/

The gas not appearing (yet) on your IHD is a different matter.

As you say it could be simply because it has not synced yet.
You could ring customer support and ask them to have an “Xjoin” command issued to the communications hub.
However depending on how long it is since the new meters were installed they may tell you to just wait a while to see if it sorts itself out.
The initial setting up of new meter’s communications can sometimes take up to six weeks to complete.

(It is also possible that the gas meter is simply unable to communicate by Zigbee wirless with the hub on top of the electricity meter, that can happen if the gas and electricity meters are too far apart, or if there is something like thick walls or a metal cabinet, etc. between them.
it’s probably much to early yet to say if that is the case, but if it is then there are some possible solutions that might help).