Your discussion topic seems to be a bit misleading, and does not totally match your actual problem.

Firstly, iPAD Wifi do not ALWAYS get turned off when the screen is locked.      For example, for the normal multi-minutes screen auto-lock set up by device user to save battery power, Wifi won't be disabled and MDM can still remotely control the device.  Similarly, enabling latest "MDM lock mode" will also not force the Wifi hardware off.

When an iPad has its Wifi disabled is defined by Apple, and implemented in Apple firmware based on device power-saving/security reasons, and is currently NOT CONFIGURABLE by any MDM software. The behaviors vary for different iOS firmware versions. 

Now, returning to your problem of locked DEVICE due to "expired password policy", I did some tests on our v11.2.x Wifi-only iPAD's to emulate your problem.   The Wifi did get disabled when password get expired. Actually, similar problems have been reported in Apple's and many MDM support web-sites for over 3 years.  Some reported fixes that might get a locked device connected back to the MDM server to receive an unlock command include:

1. USB-to-Ethernet adaptor plus powered-USB-hub or Apple's USB Camera adaptor  (e.g. see https://lifehacker.com/hack-together-a-way-to-connect-an-ipad-or-iphone-over-e-1770270331)

OR

2. connection to a MacOS machine with Lightning/USB cable  (i.e. use tethered connection.  See https://support.apple.com/en-hk/HT207523)

The above hacks explore some other loopholes of iOS firmware and might only work for some iOS versions (say 10.x)  and device models.   I haven't tried them myself.  If they could not solve your problem, you probably are left with no choice but to force your device to DFU mode to factory reset it, and temporarily stop using "time expired password policy" for all your Wifi-Only devices until Apple comes up with new firmware that fixes their "interesting" design choices.  If you need to remotely lock a suspected lost device, use the more recent MDM lost mode.