FIOS slow *except* with G1100

Wondering if anyone's seen this before...

I have 100 Mb/s FIOS with a G1100 currently.  When I run a speed test from a computer wired to that router, I see upload and download speeds in the 90s.  All good.

But, when I replace it with my own router, uploads stay in the 90s but downloads dorp to the 30s.  If I connect the computer directly to the ONT, I still only get downloads in the 30s!  And conversely, if I connect the ONT to the G1100, the G1100 to my router and my router to the computer, I get speeds in the 90s again.

So, what could be the reason why I can't get good speeds without the G1100 in the mix, and how can I fix it?

Thanks!