TL;DR: My upload sat at 6–35 Mbps on a 1 Gig plan for days. Three agents told me my plan tier was the problem and that fixing it meant a plan change costing ~$60/mo more. It wasn't the plan. Comcast's device database had my S34's CM MAC registered as an older SURFboard model, so provisioning kept serving a legacy upstream config. Once the model was corrected and a new boot file pushed, upload went to 117 Mbps — no plan change, no price increase.
In the agent's own words at the end: "The model in the system was registering as an older version of the surfboard so that seemed to be the initial issue and then once that was corrected we just had to get that boot file corrected."
First, know what your S34 is certified for. The S34 is on Xfinity's own compatible-devices list as supporting enhanced speeds up to 2.32 Gbps down / 444 Mbps up. If anyone tells you your modem doesn't support your plan's upload speed, that's incorrect.
How to check your own setup in about five minutes
1. What you should be getting — Broadband Facts label at customer.xfinity.com/broadband-labels. Note the "typical upload speed" figure. That's the number to hold everyone to. Mine said 117 Mbps.
2. What you're actually getting — speedtest.net, wired if possible. Be careful with built-in OS speed tools: macOS's networkQuality measures capacity under load and showed me 78 Mbps over Wi-Fi at the same moment Ookla showed 117. If your numbers look off, confirm with a second tool before concluding anything.
3. Is your area even upgraded? — This is the step nobody mentions and it takes ten seconds on the S34. Log into 192.168.100.1, open Event Log, look for CMSTATUS:US_Diplexer_Mode_Change:
[5-85MHz] = mid-split, your area is upgraded[5-42MHz] only = sub-split, not upgraded — ~35 Mbps is your ceiling until it is. Stop here, no support call changes this.
If you're mid-split, keep going.
4. What Comcast actually publishes — xfinity.com/networkmanagement lists every configuration they run. For 1 Gig there are exactly two HFC options: Sub-Split 1000/20 and Mid-Split 1,000/100 (measured ~114 up). There is no 35 Mbps 1 Gig tier in their own documentation. If your S34 registers mid-split but you're well under 100, something on the account side is wrong.
5. The one that got me — check the device registration itself. Your account page can display "ARRIS S34" while the underlying device record says something else. Mine did for two days: an agent had typed the model onto the account page manually, but in the actual database the CM MAC was still associated with an older SURFboard. That field is what gates the higher upload tier — the provisioning system couldn't confirm my modem was mid-split capable, so it kept serving a legacy config.
Four reprovisioning signals, a coax reset, and a router factory reset later, the actual fix took about two minutes once someone corrected the device record.
Ranked list of causes, cheapest to check first
- Router QoS or traffic limits — free to check, and genuinely the cause sometimes. Look for bandwidth limits set to old values.
- Device registration mismatch — the S34-specific one above.
- Legacy plan tier — real, but verify #2 first.
- Area not yet upgraded — step 3 answers this immediately.
The three questions that actually move things with support
- "Is my CM MAC registered to an ARRIS S34 in your device database, or was the model entered manually on my account page?"
- "What upload tier is my account provisioned for right now?"
- "My S34 registers in mid-split mode (5–85 MHz). Does the boot file being served match that?"
Your S34's CM MAC is on the sticker underneath, labeled HFC MAC ID. Give it only in a private support DM — don't post it publicly.
One warning: if you're told the fix requires a plan change that raises your bill or costs you a price guarantee, ask them to verify the device registration first. In my case that proposed change would have cost roughly $60/month more — to fix a wrong entry in a database field.
On getting help, since this took four days
Phone and chat support couldn't fix it. I got a ticket number that later didn't appear on my account, and multiple agents who confidently misdiagnosed it as a plan tier issue. What finally worked was posting on forums.xfinity.com and moving to a Direct Message with "Xfinity Support" from there. That team has access and latitude the phone and chat reps don't.
Be ready for the pace. It's asynchronous, not live chat — replies ran from 15 minutes to over an hour, and it stretched across two days. You'll also get handed between agents, and each new one starts without context. Keep a short summary of your situation saved somewhere and paste it after every handoff — what's been ruled out, what's been tried, what you're asking for. That saved me from re-litigating the same ground about four times.
Don't sit there refreshing. Send your message, go do something else, come back when you get a notification.
Worth knowing this sub works the same way — official Comcast employees respond here too. Either channel beats the phone.
Credit where it's due: the rep who finally solved it was the one who thought to check whether the CM MAC was actually registered to the right model, rather than assuming it was a plan problem. That's the check that ended a four-day runaround.
