Airport Extreme 3rd Gen dropping packets when connected to ISP's modem via Gigabit Ethernet. Works well in about 1 restart out of every 5 and lasts for about a day. I'm at my wits end, please I need help!

I moved to a new place and brought my 2009 Airport Extreme 3rd Gen (firmware 7.8.1) with me. It hasn't had any problems in over 10 years in my previous 3 places, but with my new ISP and its modem, I'm at my wits end trying to find what's wrong.

The problem:

In about one of every five restarts, when I ping the router while physically connected to the Airport Extreme via GE, I see this:

64 bytes from 192.168.100.1: icmp_seq=216 ttl=64 time=0.537 ms

Request timeout for icmp_seq 217

Request timeout for icmp_seq 218

Request timeout for icmp_seq 219

Request timeout for icmp_seq 220

64 bytes from 192.168.100.1: icmp_seq=221 ttl=64 time=0.655 ms

64 bytes from 192.168.100.1: icmp_seq=222 ttl=64 time=0.497 ms

^C

--- 192.168.100.1 ping statistics ---

223 packets transmitted, 121 packets received, 45.7% packet loss

round-trip min/avg/max/stddev = 0.459/0.602/0.997/0.092 ms

The fiber optic modem (Huawei HG8245H) has 4 GE ports, is set up in the living room where the fiber comes in, and is set as DHCP server and gateway. The Airport Extreme is set to Bridge Mode. I've already tried physically connecting them together using every combination of ports. Ports are OK, no luck.

I've also cabled the rest of the house with Cat5E cables and tested the cables are good: I've connected Mac and PC laptops directly to the modem and they have zero loss. Connecting the Airport Extreme to the modem on every room (and connecting laptops to it) yields the same result.

Continuously restarting the Airport Extreme will sometimes work (in about one out of five restarts) and things will look like this:

64 bytes from 192.168.100.1: icmp_seq=177 ttl=64 time=0.487 ms

64 bytes from 192.168.100.1: icmp_seq=178 ttl=64 time=0.497 ms

64 bytes from 192.168.100.1: icmp_seq=179 ttl=64 time=0.542 ms

64 bytes from 192.168.100.1: icmp_seq=180 ttl=64 time=0.473 ms

64 bytes from 192.168.100.1: icmp_seq=181 ttl=64 time=0.710 ms

64 bytes from 192.168.100.1: icmp_seq=182 ttl=64 time=0.699 ms

64 bytes from 192.168.100.1: icmp_seq=183 ttl=64 time=0.574 ms

^C

--- 192.168.100.1 ping statistics ---

184 packets transmitted, 184 packets received, 0.0% packet loss

round-trip min/avg/max/stddev = 0.444/0.646/8.344/0.577 ms

When things are right, they stay like that for about a day and then it starts dropping packets like crazy and not connecting to anything. It's so bad, the Airport Utility doesn't even work when connected to the Airport Extreme's own wifi network.

I've performed a hard reset of the Airport Extreme about 4 times already. I thought decade old preferences could be to blame, but there has been no difference.

I've tested with only the modem, and one laptop connected physically to the Airport Extreme (just 3 devices), and the problems start the moment the Airport Extreme has been powered on, so I've ruled out medium saturation and buffer bloat.

There are no options to tweak LAN ports or IP framing or anything on either the ISP modem or the Airport Extreme, so I can't test that. Logs on the ISP modem and the Airport Extreme (using Utility 5.6.1) show nothing out of the ordinary.

I don't know what else to do. Can someone here help me out? Maybe it's time for a new router?