I've been using LR Mobile on my iPhone for nearly a year and was quite happy with it. Today I've finally payed for a premium account, but when I opened LR app it showed zero photos. The LR app still occupies around 30 Gb of iPhone's storage, so the catalogue was not purged, but I can't find a way to re-open it.
Please do not uninstall the app. Please try the advice on this page to see if you can recover images from the Lightroom app. If this help works and you can recover images from your mobile device, then please re-import them into the account that is now subscribed:
I managed to solve the issue on my own, although the solution is quite close to the one you suggested. I used iMazing 2.12.3 on my PC and went through the following steps:
1) open iMazing,
2) attach the iPhone,
3) open the Apps menu and go to Lightroom,
4) there are 2 catalogues in the Lightroom documents "root folder" (seems like LR mobile created a new catalogue and switched to it, leaving the old catalogue untouched). Rename the old one (change folder's name) with the name of the new catalogue;
5) disconnect the phone properly and open LR mobile.
So I've already synced my mobile photos with Adobe CC and downloaded them back to the PC with LR CC.