I'm not sure whether the certification guidelines for wifi and cell devices are the same.

For Wifi it's surprisingly strict:

• Every antenna+transmitter configuration has to be certified separately (that's why Lenovo and other laptop vendors have Wifi card whitelists and refuse booting with uncertified chips installed).

• The software that directly drives the hardware must be certified to conform to the transmission power limits etc.

For DD-WRT and others neither is a problem, because the hardware combination has been certified by the router vendor, and DD-WRT uses the wifi chip vendor's firmware blob to drive the hardware, which is certified by the vendor.