Speedtest Fast, Wireless Devices Very Slow

Just had ATT Fiber installed last week and running on their 300mbps plan.

Per Speedtest (Ookla) my wired computers are getting 300+ down while my wireless regularly show 75-90ish mbps (sometimes 100+). That being said, when browsing the internet with our wireless devices (iPad and iPhones) the browsing is very slow. The blue bar shows up at the top of the Safari web page like it is loading the new page but takes a while before changing to the new page and, often times, the blue bar is frozen. Interestingly, when I use a different speed test site, I get much slower speeds (sub 20mbps). 

Secondly, I have an office on the same floor as the BWG210 (opposite side of the house, but not far at all) and I am unable to get any reliable wifi in that room - sometimes none. The modem is not enclosed in a cabinet or closet. Prior to moving to ATT, I was using a Netgear Nighthawk router that was tucked in a closet upstairs, but I was still able to get wifi in that downstairs room. 

1) I am considering adding either a wireless router after the BWG210 with ip pass through to handle the wifi capabilities. Will this help - for both issues above?

2) Another option I am considering is a mesh network (Velop, eero, etc), but I also have Sonos and, from what I have read online, I would need to run the mesh network in bridge mode so that the wireless devices can "see/control" the Sonos devices when opening the Sonos app. If I do this, does this essentially mean I am still using the wifi from the BWG210 and "linking" that to the mesh devices? If so, that wouldn't resolve my problem would it?

Any other thoughts or ideas would be greatly appreciated!