Can not verify server identify

You have a very old iPad2 - which does not receive system updates.

iPad2, iPad3 and iPad mini1 can only be updated to iOS 9.3.5 (WiFi Only models) or iOS 9.3.6 (WiFi & Cellular models). Apple ended update support for these models in September 2016; these models are obsolete.

As your iPad has not received any updates since becoming obsolete, the Certificate Store on your iPad (upon which your iPad relies in order to verify the identity of many websites) has also not been updated. Beware of clicking links that cannot be verified - as you may be accessing a malicious website. Given that you iPad is now unsupported - without benefit of any security updates, using your iPad to access sensitive or personal material is unsafe.

Notwithstanding the above, it may be possible to update a key security certificate using Safari. From Safari, open this link on you iPad: https://letsencrypt.org/certs/isrgrootx1.pem

Follow prompts to install the certificate on you iPad.

You are very welcome.

You would be well advised to take heed of the warning concerning the overall security of your iPad. If only from the perspective of security and safety, you really should consider replacing an obsolete/unsupported device with a supported model.