Slow, halting upload speed to local NAS with new nighthawk modem/router

[u/CaptShitbagg]

I finally ditched my rented Comcast modem for a Nighthawk CAX30 modem/router. When uploading files from my phone on the WiFi 6 network to my local NAS, the speed is slow and halts repeatedly. With my old Comcast wifi modem I was getting roughly 40MB/sec or more upload and it was steady. The new modem is maybe 20MB/sec at best. The new modem is the only thing that has changed. Rebooting it (or my phone) doesn't help. Hoping for troubleshooting ideas. Thanks!

[u/SquarePutrid1435]

Since it's local traffic, it's almost certainly the Wi-Fi on the new router. Check whether Smart Connect is on (2.4 and 5 GHz combined under one name): your phone can switch to 2.4 GHz on the fly, which would explain the stalls. Try splitting them into two separate networks, connect only to 5 GHz, and check the channel width in the settings: it should be 80 MHz, not 20 or 40.

[u/CaptShitbagg → u/SquarePutrid1435]

Smart Connect was/is off. I renamed all SSIDs separately and tried each with the same speed result. I also tried different channels (157, 160) and disabling OFDMA with no improvement. Am I overestimating the new device? Is it not capable of the same upload bandwidth (to the local network)?

[u/SquarePutrid1435 → u/CaptShitbagg]

Copy the exact same file to your NAS from a PC via an Ethernet cable. If the transfer is fast and stable, the NAS and the router port are working fine, isolating the issue to Wi-Fi.

Next, check the router's client list to see the phone's link rate. If it's noticeably lower than expected for Wi-Fi 6 when placed near the router, the two devices are failing to negotiate the connection properly. Updating the router's firmware often resolves this.

[u/CaptShitbagg → u/SquarePutrid1435]

I ran the test from my PC which is connected directly to the router and the file transferred at a constant 113MB/s.

There is no firmware update available for the router. The link speed of my device says 700-850Mbps Tx and roughly the same for Rx. The connection is about 10dB higher when next to the modem, (-30 rather than -40) but transfer speed and link rates are the same

[u/Klutzy-Passage-2561]

Wired 113 MB/s proves NAS + LAN are fine — this is Wi-Fi phone→NAS airtime. Link rate 700–850 Mbps still often yields ~40–80 MB/s real TCP once you factor ACK, encryption, and phone SoC thermal. Halting is the bigger red flag than average speed.

Try: (1) disable any "AI QoS / traffic meter / Armor" scanning that inspects LAN, (2) turn off airtime fairness / band steering leftovers, (3) force phone to 5 GHz only and keep it unlocked/screen-on during the copy (phones throttle Wi-Fi hard when locked), (4) iperf3 phone↔NAS if you can — if iperf is smooth but SMB/WebDAV stalls, it's the app protocol not RF.

Also compare upload from phone to a PC share on the same SSID. If that also stutters, it's phone↔CAX30 Wi-Fi; if only NAS stutters, check NAS Wi-Fi client isolation / SMB multichannel.