What if you choose one device instead of all devices, and change from daily to other time?

modem not identified? https://www.tp-link.com/en/faq-270.html no internet? https://www.tp-link.com/en/faq-269.html

MR400 is compatible with tether, if the app cannot find the device, check the link. And you need to login to tplinkmodem.net to enable remote management. https://www.tp-link.com/en/faq-1580.html If...

Yes MR3420 can use WAN connection with 4G backup. As for how long it takes to switch to online network, never try that. I assume it shouldn't take long otherwise there is no meaning using backup

no electrical outlet. just connect it to computer

If your printer is not listed, that means tplink does not test that model with the WPS510U, so cannot say it will or will not work.