If I get your meaning correctly, the USB port on the WNDR3400 is only for connecting to USB flashdrives and hard drives. So No, you can't "USB tether" if you intended to use the USB port that way. I'm not familiar with the Mifi5510L hotspot, but I would say that if it has regular regular LAN output ports like on the WNDR3400, then there shouldn't be any harm in connecting them via RJ45 ports
Aug 07, 2017 DD-WRT About. DD-WRT is a Linux based alternative OpenSource firmware suitable for a great variety of WLAN routers and embedded systems. The main emphasis lies on providing the easiest possible handling while at the same time supporting a great number of functionalities within the framework of the respective hardware platform used. How to Unbrick a Netgear WNDR3400 N600 Router after a Bad Jan 19, 2014 WNDR3400v3 | N600 WiFi Router | NETGEAR Support
dd-wrt - Browse /betas/2011/12-09-11-r17990/broadcom_K26
Netgear WNDR3400 N600 Wireless Dual Band Router: Homepage. Netgear WNDR3400. Wikipedia. Netgear. WikiDevi. WNDR3400v1. dd-wrt. Supported 17567 OpenWRT. Not Supported Tomato. Not Supported TomatoUSB. Not Supported Gargoyle. Not Supported
Jun 18, 2020
Solved: Does an WNDR3400 work as an wi-fi repeater if the dd-wrt will work on the wndr3400v1 and offers repeater and wifi repeater modes and wpa2 with aes because easy to over power the radios on wndr3400v1 if set the tx power other then auto. If interested I have the dd-wrt firmware firmware number or a copy on a computer.