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If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network - Apple Support
If you have a VPN App or profiles installed, or a security App such as Norton, these may be interfering with DHCP and preventing your device from obtaining a valid IP Address for your network. The resolution is relatively simple; may only need to temporarily disable or remove the Security App and/or VPN.
DHCP is frequently used to automatically obtain an IP Address from your WiFi Router. Until your iPad has a valid IP Address to connect to your network, you will not have an internet connection. Disabling or removing VPN and/or the offending security App should allow DHCP to operate normally - allowing connection to your network and the internet.
If this resolves connectivity issues, the VPN and/or security App can be reinstated after the connectivity problem has been resolved.
This support page will describe the more secure (recommended) settings for WiFi:
Recommended settings for Wi-Fi routers and access points – Apple Support
It is possible that your older devices cannot use the most up-to-date security protocols - especially WPA3. As such, you’ll need to configure settings that are mutually supported by both your devices and your router.
Whilst WPA3 is the newest standard, you should aim to use WPA2 (AES-PSK) as a minimum - this most likely being supported by your older iPad. You might expect to see privacy warnings from some newer devices, but of itself these are largely informational.
