yes I did the same. There seems to be no way it will let us uninstall and it has sent the text to all my contacts. I suppose i'll have to bite the bullet and reset to factory settings.
Think in these situations it will be necessary to factory reset the device. The advice is to only restore a back up that was created before you installed the scam app. For future reference ensure Google Play protect is enabled https://support.google.com/googleplay/answer/2812853?hl=en-GB
See more from the National Cyber Security centre https://www.ncsc.gov.uk/guidance/flubot-guidance-for-text-message-scam
I had my elderly neighbour over with the same issue, message app had changed to the DHL one. Every time I tried tapping the 'back' button (samsung S5) for a split second it showed me that it was asking if it was ok to make the DHL message the default message app. After remembering where the 'cancel' was on the screen I kept on going in to the message app and then 'back' (the '<') and trying to be super fast to hit the spot where the 'cancel' is .... eventually I got it but it still didn't work.
Running a security check didn't pick anything up.
She had never done a back up and reading up on it, signing in to anything once the DHL app had installed was a security breach risk, creating an account to do a back up and resetting the phone to then lose all contacts and pics wasn't an option she wanted to face.
Solution which worked without needing to do a reset :
Went in to the settings - Apps - then the 3 dots and found the 'default apps' - messages (which showed the DHL as default app). When I tapped it it said that 'you can't change system processes' .. or something like that. Once again though there was a split second that I could see the options available.
I went back in to settings - apps - (not the 3 dots this time) found the DHL spam app, went to uninstall it and again was confronted with the "cannot change bla, bla, bla" message but again, the split second, and I do mean a very, very quick split second, where I could see the option to delete before it automatically went back to settings.
Repeat the process and remember where the option to delete the app was on the screen for that split second and to try and tap that spot before it disappeared. It eventually worked and I was able to delete the app. Message app returned to normal and on restarting the phone there was no sign of the DHL app.
All up it took about 25min fumbling around but got there in the end. Her phone was a Samsung S5 so not sure if it'll be the same for everyone but hopefully this 'split second' viewing of options before disappearing off screen works for others, got to be quick with your screen tapping.
I'm not sure if you found a solution but I figured it out. Basically you want to go to settings, go down to privacy, go into permissions. It should show that the DHL app has full permission of your phone. Disable it. Then you'll be able to uninstall it. Fingers crossed this helps if needed.