Update
Verizon sent me another Verizon MI424WR router.
Out of the box. Connected. Only TWO devices connected (this device - an Apple laptop that I'm typing this message on) and one of the two Dell laptops in question. No more wifi-palooza on my old network of multiple devices.
Actually WORSE wifi connection speed on this device (the Apple laptop), via Ookla and other speed test sites than with the previous Verizon FIOS router.
And, SAME persistent issue on the both Dell laptops.
My only other idea is to connect some sort of WiFi access device (such as an Apple Airport, Google/Amazon "mesh" WiFi device) to either the old or new FIOS router to give these Dells a non-FIOS wifi access point. As discussed above, these Dells did fine wifi connection speed-wise when connected to an iPhone hotspot running on TMo's network, an iPhone hotspot running on Verizon Mobile's network and a Mifi device running on AT&T's network. Perhaps using some sort of WiFi access device (such as an Apple Airport, Google/Amazon "mesh" WiFi device) to either the old or new FIOS router might "avoid" the problem. In substance I'll be "away" from FIOS's WIFI and connected via another's WiFi (albeit on a wired connection to the FIOS router - which, as discussed above, provided a fast connection to the Internet).
Switching back to "old" FIOS router as this new one offered no improvement/fix and don't see the need to change every device I own to connect to the "new" device.
Update:
Bought this
Delivery on Tuesday. Fingers crossed...