If you find yourself getting a PDP Authentication Error on the iPhone (any iPhone for that matter) it simply means your iPhone isn’t correctly configured to authenticate with the data service of your carrier.
While there are some basic things you can do to initially troubleshoot the issue such as restart your phone, reset network settings etc these usually won’t do anything.
The most likely reason is that your APN is wrong in which case your phone doesn’t know how to communicate with the data provider. Unfortunately in many cases Apple doesn’t let us change this so many mobile providers let you download an APN Profile from their website to install on your phone. Make sure you do this at the airport or where you buy your SIM while you still have wifi.
For Example B Mobile in Japan with the Visitor SIM it won’t work unless you install the profile. https://www.bmobile.ne.jp/english/setting_ios.html
Other carriers will have a similar download page or manual APN settings. You can access the manual APN settings via Mobile > Mobile Data Network (or Mobile > Choose Plan > Mobile Data Network on phones with ESIM capability). If you don’t see this option then it’s disabled for your SIM type and the only way to set it is to download a profile from your carrier or to make one manually using this site (IG you cant find one from your carrier but know the APN details which are usually published all over the internet) https://www.unlockit.co.nz/fw/