You can first try to install UT again with UBports Installer. Don't check "Wipe" neither "Bootstrap partition". Then, you will restore Ubuntu Touch with all your data.
If this doesn't work, you can try the following method, but warn that it's quite dangerous if you delete any wrong file, and that can lead to a non working device. So do this ONLY if you know what you are doing and assume the risk....
First, list all residues of the OpenStore:
$ sudo find /opt -iname *openstore*
and
$ sudo find /opt -iname *openstore-team*
Note all related packages (warn that it's not attached to another app)
Then delete all those related files like:
$ sudo mount -o remount,rw /
sudo rm -rf /opt/click.ubuntu.com/.click/users/phablet/'openstore_file'.ubports
(Replace 'openstore_file' with the name of the package you have listed / check the exact path as well).
Repeat this with all the remaining files related to the OpenStore.
Then, Re-install Ubuntu Touch using UBports Installer without checking "Wipe Data" nor "Bootstrap partition".
OpenStore should re-appear at the first reboot.