Xfinity Fiber Installed — Great Wired Speeds, But Wi‑Fi Is All Over the Place. Need Advice.

A few days ago, an Xfinity tech came out to install fiber, an ONT, and the new gateway. When he left, everything looked great—Wi‑Fi speeds were hitting around 800 Mbps on our 1 Gbps plan, and the wired connection was perfect.
Since then, the Wi‑Fi has gone completely inconsistent. I’ve been checking speeds throughout the day, and they bounce anywhere from 50 Mbps to 300 Mbps, sometimes dropping altogether. Meanwhile, if I plug directly into the XER10, I get the full 1 Gbps with no issues, so the fiber and ONT seem fine. The problem is clearly the wireless side of the gateway.
It’s not my devices. We still have our old provider active while we test Xfinity, and those same devices pull around 700 Mbps on the old network with zero drops. So the issue is isolated to the Xfinity gateway’s Wi‑Fi performance.
Here’s what I’ve tried so far:
•  Restarted/rebooted the gateway multiple times — one reboot helped briefly, but speeds dropped again.
•  Renamed the 5 GHz band to force devices onto it — no improvement.
•  Tried logging into the gateway — the default admin/password credentials don’t work.
I’m hesitant to do a full factory reset because I don’t want to wipe everything unless that’s truly the next step. But right now, Xfinity is not looking great compared to what we already have.
For context, this is all on a Quest 3s household with a mix of devices, and the inconsistency is noticeable across all of them.
At this point, I’m trying to evaluate whether Xfinity is worth switching to, and so far… it’s losing. Any advice or next steps would be hugely appreciated. Thanks in advance!