Since you have a WRT54G, you can turn it into a 10/100Mbps switch and place this between the modem and the wan port of the wrt1900ac. This should limit the connection to 100Mbps max. Connect the wrt1900ac wan port to a lan port of the switch, and connect the modem lan port to another lan port of the switch.
There are various tutorials on the web on how to turn the wrt56g into a switch. The basic important steps:
-backup your wrt54g config into your computer
-change the wrt54g lan IP into something that doesn't conflict with your wrt1900ac ip.
-disable the dhcp service in wrt54g
-don't plug anything to the wan port of wrt54g _________________
Netgear R7800 kongpro 19.07 20190616 || Netgear R7000 35550M kongac (Client Bridge=5GHz, AP=2.4GHz)
Linksys WRT32X davidc502 OpenWrt || Linksys WRT1200ACv1 Gargoyle 1.11.x
Linksys WRT1900ACSv2 dd-wrt 39956
It's just a temporary setup, just to prove your theory that the wan only works with 100Mbps. Once you have proved/disproved that, you can put the wrt54g back to original config.
Another way to force the link to 100Mbps max is that gigabit speed need all 4 twisted pairs of the cable. If you cut one of the pairs at the side, you will be left with less than 4 pairs and it won't negotiate to 1Gbps. From the pinouts, you can get a cheap cat 5 cable and cut wires connected to pins7&8.
That's a great idea.
I'll use a cat 5 cable and that should force it to use 100mbps.
A properly wired cat5 will carry gigabit with no problems. Not best practice, but I have done it many times. Might be an issue at longer distances.
If you really want to force a link into 100mbps mode you need to actually snip the extra pairs in the cable that gitabit uses. Either slit the cable and cut the pairs going to the appropriate pins, or if you have a cable crimper, just crimp a new end with those pins disconnected.
The pins to disconnect are pins 4,5,7 and 8. On a properly made ethernet cable these will be the blue and brown pairs
I was able to clip an Ethernet cord based on the diagram but to no avail. All devices that i connected with that clipped cable connected properly at 100mbps. However, when i connect my modem to my wrt1900ac with the modified cable, i still get 0.0.0.0.
Back to my original thought, can you limit the wan port speed on the wrt1900sac router running ddwrt?