custom channel _mask does not work properly in wisun

[mcuxmx]

Description of defect

I use wisun EU868 regional parameters (regulatory domain: 1, operating class: 1, operating mode: 0x1b).
If all channels are enabled(all channel mask value as 0xffffffff), the border router and the router can be successfully connected.
But if only two frequency bands are used (864.0-865.0MHz, 866.0-868.0MHz). I change the channel_mask to (0xc007fc00, 0x0001ffff, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0), the border router and the router will not be able to connect.

Target(s) affected by this defect ?

border router: stm32f769 discovery + s2lp
router: stm32f429 nucleo + s2lp

Toolchain(s) (name and version) displaying this defect ?

N/A

What version of Mbed-os are you using (tag or sha) ?

mbed-os-6.13.0

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

mbed-cli 1.10.4

How is this defect reproduced ?

always

[0xc0170]

@mikter can you please review?