If you did not get to a resolution, some more information could be handy.
I had to tweak my MacOS stack & FiOS router mercilessly to get advertised performance. The good news is it is possible. Even though my AV specialist told me it's 'normal' and can't be fixed (and Verizon won't help with anything once it passes the router, they claim they are only responsible to guarantee the advertised speed to the router.
1) are you wired for ethernet or MOCA (coax) from your on-premise-equip terminal (OPE)?
MOCA can behave differently, ethernet can be easier to deal with
2) what model and firmware revision is your fios router?
Note that ActionTecs are old, twitchy and past end of life. but I have one and got it to work... so don't freak;
3) connect to the admin account on the router (via its web interface) on your network
note any active packet filtering rules or port forwarding rules present, or specific services blocked
report those here
I recall I had better results after enabling certain external pings (which were disabled as a security measure to reduce attack surface)
4) check the MTU packet size if the router exposes it
5) check the same value on your mac and note any difference
your mac should be using a size <= the router
I hope this will lead you on the right path...
if you post back, I'll see if I can help further
Finally, one trick to try
if you have access to an external VPN service/subscription that operates at the system admin level on your mac...
launch that and connect your mac so it is connected through the VPN (and all its traffic will be encrypted, and Verizon can't peek at your packets) [note a corporate VPN may not work]
then run your speedtest and see what you get.
I'd be very curious. I know for a fact verizon interferes with some services (including VPN!) based on that, and when you can encrypt an established connection, their interference goes away...