RT-AC86U w/Asus-Merlin, Slow Speed - How To Get Max ISP Speed?


Occasional Visitor
Hi everybody!

My ISP recently upgraded our max DL speed from ~100 Mbsp to about 300 Mbps. Unfortunately, using Speedtest.net, I am still maxed at about 95 Mbps, both wired and wireless. I am hoping you enthusiasts here can help me. You have always been so helpful in the past!

Some details:

I have had a Asus RT-AC86U running Merlin for a few years. I have a Pi-Hole, which does seem to be working, connected via WiFi. My main PC is connected to the router via Cat 6a. All the cables are Cat 6a. O/S is Windows 10 LTSC 2021. Mother board is an ASUS MAXIMUS 8 HERO ALPHA, which was awesome 6 years ago. It has latest BIOS/firmware.

What I did already:

When I had techs from my ISP on the phone, they noticed that the modem (MOTOROLA MB8600 DOCSIS 3.1 Cable Modem) was connecting to the router at 100 Mbps max. When I bypassed the router and connected modem straight to my PC, speed stayed at less than 100.

Then I got a "free" DOCSIS 3.1 modem from the ISP and hooked it up. Speed stayed the same <100Mbps through the router, but jumped to 340 Mbps when modem connected straight to the PC!

Then I updated the RT-AC86U to latest Merlin (from 384.18 to 386.7_2 - a pretty big jump). I followed some of the unofficial guides here as I always do (thanks so much @L&LD and others!). I flashed, then did the factory reset, before doing some configuring. Still no good.

Where I am at:

Unfortunately. I am still at less than 100 Mbps through the router. The router "Status" tells me that the LAN port to my PC is 1 Gbps, but the WAN port is 100 Mbps. There are no other wired ports connected to the router. Everything else is WiFi.

So, are there any settings I can try? Might I have a bad router? Could the Pi-Hole be set up wrong? I don't have a lot of knowledge in networking, but can follow directions and guides.

And guys/gals, straight up: even if you can't help me on this, I want to say thank you for all the written assistance this forum has provided me and tens of thousands of others over the years. You probably have no idea how important this site is.

Thanks so much!
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Very Senior Member
but the WAN port is 100 Mbps.
The WAN port from your router is showing as 100Mbps? If so then I would think that's your problem there.


Regular Contributor
Try a different Modem->Router Ethernet Cable. Maybe the one you are using is borderline and just so works at 1000mpbs between the Modem and the PC but not between the Modem and router.
I suggest powering both Modem and Router off, then swapping the cable and powering them back on and see if it helps (fingers crossed)


Occasional Visitor
OMG, that was it! It was the "modem to router" cable. After decades of "checking the cable" and it not being the cable... it was the cable. There is a lot of LOL here at the moment!

337.45 Mbps!

Now, my earlier test of the Motorola modem. I wonder if the modem is bad. Or could a few years of "bad cable" damage a modem? The bad cable did not show up before 'cause I did not have fast enough speed to notice.

Also, the "bad" cable was from Cable Matters, which I still think is a good brand. I have been getting them for years. So I extra doubted a cable problem. Any advice on cable brands?

Thanks all! I can't believe it!

Edit to add:
Yeah, the router status is reporting the WAN as 1 Gbps now.
Since the original Motorola modem still had less than 100 Mbps speed when plugged straight to the PC (and bypassing the bad cable), I wonder if it is bad, too.
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Occasional Visitor
Wait, it was not the cable! The cables are fine (sorry to slightly defame you, Cable Matters).

As I was digging through the (embarrassing) mess of extremely hard to access cables to pull out what I thought was the offending wire, I saw that I had run cables in to and out of my CyberPower 1000PFCLCD UPS. It was supposed to (I assume) protect against surges coming through Internet. THAT was limiting my connection! A UPS! I had no idea. I pulled out my old manuals on the UPS, and the papers I have don't claim what data speed rating it has. A slightly newer version of the same model online claims 10/100/1000 for what its worth. Mine is from around 6 years ago.

So a note that running Ethernet through surge suppressors and UPS's can affect connection speed (and probably other problems), and it might not show up till your Internet speed increases. I am probably the only one here who had not thought of that. I had speed tested it in the past.

Also, I have a call into my ISP to retest the Motorola modem as I may (or may not) have still had the UPS inline when I tested the modem. Will update here later with that piece of self-deprecating entertainment.

Edit: And the modem is good! I can't believe my oversight. But all is well now! Thanks everyone!
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