RT-AX86U slow download speeds

[GW4C]

I've had this router for a little over two years and it has functioned perfectly until this last month. I have 2gig symmetrical fiber internet and when using the router's built in speed test, I was getting ~1,800mbps download and ~2,000mbps upload until mid-March. For the past couple of weeks, the speed test is showing ~400mbps download and ~2,000mbps upload. I have no idea why the download speeds are so slow. A technician from the internet provider stopped by and confirmed that the ONT is flashing a green light which means that the receiving client (my router) is sub gig. They connect the ONT to two different routers they brought with them and both of them were showing ~2,000mbps download speeds. So it has to be the router. Does anyone have suggestions on what I can do to troubleshoot this issue?

Some additional details:

• The ONT is connected via Cat5E cable to the router's 2.5gb wan port.
• The router firmware version is 3.0.0.4.388_24231
• I have tried rebooting the router and ONT.
• I have not tried to factory reset the router.

[GW4C]

Correct. I have a USB-C to 5gbps ethernet adapter and I was able to get advertised speeds (2gb download and upload) when connected directly to the ONT. I'm curious -- what do you think cloning the MAC will do for the router?

[jzchen]

I'm suspicious of ISPs who offer routers for rent, from past experience, many many years ago.  (A time where service was sold per client, and routers circumvented that).

(Worth a shot)...

[Saltgrass]

Did cloning a MAC address help?  I would probably look at the ONT or any device that came with the install, for a workable MAC address.  Perhaps, even the Ethernet dongle.