iPads unable to connect to home Wi-Fi after update attempt

You may have tried some of this already, but is included here in case something has been missed. Start here:

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network - Apple Support

…and here:

Recommended settings for Wi-Fi routers and access points - Apple Support

WiFi/Internet connection difficulties are usually an indication that your device does not have a routable IP Address - your IP Address usually being assigned by DHCP. When problems occur, your iPad may only have a self-assigned link-local IP address that begins 169.254.x.x.

You can verify your DHCP-assigned IP Address from settings:

Settings > WiFi > [Your WiFi Network / SSID] - tap the blue “i” icon - [IPv4 Address] IP Address

If you have a VPN App or profiles installed, or a security/AV/firewall App such as Norton, these may be interfering with DHCP and preventing your device from obtaining a valid IP Address for the WiFi network to which you are attempting to connect. The resolution is usually simple; if installed, you may only need to temporarily disable or remove the Security App and/or VPN - and then restart the affected device(s).

DHCP is frequently used to automatically obtain an IP Address. 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 your issue, the VPN and/or security App can be reinstated after the connectivity problem has been resolved.

If some Apps are working as expected, while others are not - or other devices can connect to your WiFi network - you may find that a restart of your WiFi Router, followed by a restart of your iPad will clear the problem….

To restart your router, disconnect power from the router for at least 30 seconds - then reconnect power to restart. After your router reboots (which may take several minutes), force restart your iPad:

Force restart iPad (models with Face ID or with Touch ID in the top button)

If your iPad doesn’t have the Home button, do the following:

1. Press and quickly release the volume button nearest to the top button.
2. Press and quickly release the volume button farthest from the top button.
3. Press and hold the top button.
4. When the Apple logo appears, release the top button.

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

If you continue to have difficulties, check to see if your iPad is using the WiFi Private Address feature. This feature is intended to provide additional privacy and security when using “public” (untrusted) WiFi networks.

Settings > WiFi > [Your WiFi Network / SSID] - tap the “i” icon - Private Address

The Private Address feature is largely unnecessary for your home WiFi network; if enabled, Private Addressing can cause issues if specific MAC addresses are expected by your WiFi Router. You may need to disable this feature - and can be enabled or disabled for each individual network.

Use private Wi-Fi addresses in iOS 14, iPadOS 14, and watchOS 7 - Apple Support

Verify that you have no "security" Apps (e.g. Norton) or VPN Apps or profiles installed. If they are present, fully remove them from your affected devices - and then perform a forced restart of the devices.

Otherwise, given that both iPads work as expected when connecting via your phone hotspot, the only remaining conclusion is that something is awry with your Router or its configuration. With this, two options remain...

1) Fully reset the TP-Link Router to its factory settings - and then reconfigure only the setup parameters needed to connect with your ISP, testing connectivity with the default router settings.

2) Replace the TP-Link Router. Perhaps you have your ISP-supplied router to hand? Usually the ISP-supplied router is a "basic" model; this is often useful as a diagnostic tool.

Using MAC addresses to restrict access to a network is an old, ineffective and now largely deprecated access control measure. Reliance upon the device MAC Address is unreliable at best - as MAC Addresses are trivial to spoof and equally simple to "discover" using readily available network utilities.

The physical address used by a client device (i.e. MAC Address) is broadcast in-clear within every IP packet/frame - and can be trivially recovered from WiFi or with physical access to a wired network. As you are using WiFi, broadcasting your MAC addresses to anyone within range, MAC white-listing is ineffective. If you intend to employ network access control measures, you will need to explore considerably more secure methods - such a 802.1x with Certificates - which, for the majority of domestic applications is impractical.

Given the above - and the fact that a significant proportion of modern "connected" devices may regularly rotate MAC addresses for "privacy" (this measure being one aspect of ameliorating device fingerprinting and tracking) - you might be well advised to either (a) fully disable MAC Address white-listing for your Router, or (b) ensure that your client devices are configured to use either their globally unique (i.e. burned-in) MAC address, or a fixed MAC within device settings.

Be aware that reliance upon MAC white-listing will increase your administrative overhead - as client network settings/parameters may be changed or reset for a variety of reasons.

Addendum:

An additional point of note concerning your initial post...

Rolling-back to an earlier version of iOS/iPadOS is not possible. All System Updates are digitally signed by Apple - and can only be installed whilst the digital signatures remain valid. After release of a new iOS/iPadOS version, the immediately preceding version continues to be digitally signed by Apple - however, usually within a few days, Apple will revoke the digital certificate with which the earlier version is signed.

Perhaps consider that iOS/iPadOS updates include both functional and essential security updates. Restoring an earlier version of the OS would expose your device, privacy and data to emerging and known security threats and vulnerabilities.

Have you disabled your MAC Address access controls on your router?

If not, at least as a diagnostic exercise, I strongly recommend that you do so - and then restart your router by disconnecting power for at least 30-seconds. This will clear your router's local ARP table and reset DHCP. Next, restart each of your client devices.

Unless explicitly stated or confirmed, I can take nothing for granted.

From your earlier description, I presume that you have an older model BT Router. I make assumption that the router is currently configured to use a combined SSID (network name) for both 2.4GHz and 5GHz WiFi bands.

Again, for diagnostic purposes, I suggest that you split your WiFi such that your 2.4GHz and 5GHz WiFi have a different SSID for each band - effectively creating two independent WiFi networks for your Router. For security, select WPA2 - and nothing else.

With separate SSIDs configured, test your troublesome duo in this configuration.

With MAC filtering fully disabled, this is one area that can now be dismissed from the equation.

Did you perform a restart of both the router - and the your client devices - as directed after disabling MAC filtering?