300mbps speed loss for over a month, no help

This started around 12 February and I've not gotten any answers as to why. Prior to that date, I was getting 1375-1425mbps, no errors, no lag, etc. There was some workers, here, today, that replaced a large sized piece of equipment, outside, that'd gone bad, but after they'd completed their work and my internet came back up, I still am not getting over 1100mbps, except rarely. I own my own equipment and bought it, specifically for the higher download speed. My modem is a Netgear CM2000, router is an Archer BE550 and the motherboard ethernet port is Intel i225-V. All are certified up to 2500mbps, with the router's WAN and LAN ports having a throughput of 2500mbps. My ethernet cables are CAT 6 STP, rather than UTP. Everything I can do, I have done, to get my download speed back above the (now) 1300mbps I'm paying for. Conversations with live agents have not resulted in any change, of any kind. "Everything is fine, on our end. It must be your equipment." Can anyone help, please? I'd appreciate it, very much.

The tech that came out, on 15 Feb, replaced an outdoor cable, that is still lying on the ground. I've not gotten any phone calls, from anyone, about what other cable would be replaced. I'm past the point of being irritated and getting angry.  Additionally, before the speed drop, my iPhone hit 1384mbps using Ookla Speedtest app, via the aforementioned router and modem.