On GL-RM1 there is support for CH341 usb/rs232 adapter:
[717681.885679] usb 2-1: new full-speed USB device number 2 using ohci-platform
[717682.108223] usb 2-1: New USB device found, idVendor=1a86, idProduct=7523, bcdDevice= 2.54
[717682.108280] usb 2-1: New USB device strings: Mfr=0, Product=2, SerialNumber=0
[717682.108301] usb 2-1: Product: USB2.0-Ser!
[717682.121671] usb 2-1: ch341-uart converter now attached to ttyUSB0
On GL-RM1PE it is missing:
[ 6.689790] usb 2-1: new full-speed USB device number 2 using ohci-platform
[ 6.914895] usb 2-1: New USB device found, idVendor=1a86, idProduct=7523, bcdDevice= 2.54
[ 6.914942] usb 2-1: New USB device strings: Mfr=0, Product=2, SerialNumber=0
[ 6.914969] usb 2-1: Product: USB2.0-Ser!
[ 20.667374] dwc3 21500000.usb: device reset
Both on latest firmware.
As I will get GL-RM PRO in a few days please check there too