The most likely reason that the technician is unable to select the channel with almost no RF pollution is that it is reserved for licensed band use. Some channels are reserved for specific purposes and require a license to use. These licensed bands are typically used for critical infrastructure, public safety, or military operations, and unlicensed devices are not allowed to use them. References: CompTIA A+ Certification Exam Objectives -
220-1001: 3.3 Compare and contrast common networking hardware devices.

The next troubleshooting step that the technician should attempt is to restart the smartphone. Restarting the smartphone can help resolve many common issues, such as battery drain, network utilization, app crashes, or performance slowdowns. Restarting the smartphone can do the following:
* Clear the memory and cache of the smartphone, which can free up space and speed up the device.
* Close any background apps or processes that may be consuming battery or network resources, such as syncing, updating, or streaming.
* Reset the network settings and connections of the smartphone, which can improve the signal strength and quality.
* Refresh the operating system and the user interface of the smartphone, which can fix any glitches or
* errors.
To restart the smartphone, the user can follow these steps:
* Press and hold the power button of the smartphone until a menu appears on the screen.
* Tap the option that says Restart, Reboot, or Power off and on.
* Wait for the smartphone to turn off and then turn on again.
* Enter the PIN, password, or pattern to unlock the smartphone.
* Check the battery level and the network utilization of the smartphone.
If restarting the smartphone does not solve the problem, the technician may need to try other troubleshooting steps, such as updating the mobile OS, disabling Bluetooth, or connecting the smartphone to a Wi-Fi network.
However, restarting the smartphone is usually the first and simplest step to try.
References:
* How to stop "Mobile network standby" from draining my battery1
* Battery life: Guide to everything that affects and drains your phone battery2
* Mobile Network Standby Battery Drain; How To Fix It?3
* Get the most life from your Android device's battery4
* How to Fix Battery Drain from Mobile Network on Galaxy S235

The technician should check the MAC filtering list next as the new router may have a different MAC address than the old router. Reference: CompTIA A+ Certification Exam Guide, Exam 220-1001, Page 332