After asking around, I was told to try the Ubiquity Edgerouter 4, which is indeed a nice little router and comes with linux out of the box. The best thing is it's designed to give you full access so you can install your own linux software. Well done!

But, for some strange reason, the then wonderful people at ubiquity did not figure out that it was useful to plug a serial port adapter to monitor or connect to another device, or even to use/need external USB storage. It is of course linux, you can build your own kernel and modules, but doing it for another architecture requires cross compilers and a non trivial setup that would likely have taken me days to setup.

This is where Nils Andreas Svee (Lockhair) comes in, and as part of his CAKE project to improve networking on that device (requiring a new kernel), he built a full compiler suite for that kernel, and very nicely added USB serial modules to his build for me a few years back for the ER1.x firmware.

Fast forward a few years, and the original 1.10.11 was based on Debian 7 (Wheezy) which became totally obsolete. Getting updated packages for that ancient distro became difficult, so I figured I should upgrade to 2.x.
After doing so ( EdgeRouter ER-4/ER-6P/ER-12/ER-12P: Firmware v2.0.9-hotfix.7 31 Jul 2023 from https://ui.com/download/releases/firmware ), I of course realized it was a new kernel that needed new modules.

But then I realized that during that time Ubiquity turned evil and stopped releasing GPL required code, including the kernel, which in turn caused Lochnair to stop being able to maintain his project: https://community.ui.com/questions/CAKE-thread-gone/27de66b2-9ac2-4fcb-93f7-d02cd5785b4c

I had a glimmer of hope that the new binary kernels were close enough to the last officially released kernel source, and asked him if he could build the extra missing modules: https://github.com/Lochnair/kernel_e300/issues/2 . He super nicely agreed and they worked!

Thanks to him, I now have have a fully upgraded and featured linux route with usb-serial support and usb-storage support.

You can get the modules here: 4.9.79-UBNT Serial and USB-Storage Modules << DOWNLOAD ME

usbcore: registered new interface driver pl2303
usbserial: USB Serial support registered for pl2303
pl2303 1-1.1:1.0: pl2303 converter detected
usb 1-1.1: pl2303 converter now attached to ttyUSB0
usbcore: registered new interface driver uas
usb 1-1.2: new high-speed USB device number 4 using xhci-hcd
usb 1-1.2: New USB device found, idVendor=0951, idProduct=1666
usb 1-1.2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
usb 1-1.2: Product: DataTraveler 3.0
usb 1-1.2: Manufacturer: Kingston
usb 1-1.2: SerialNumber: 1C6F654E48EBB131D95D0ABB
usb-storage 1-1.2:1.0: USB Mass Storage device detected
scsi host0: usb-storage 1-1.2:1.0
scsi 0:0:0:0: Direct-Access     Kingston DataTraveler 3.0 PMAP PQ: 0 ANSI: 6
sd 0:0:0:0: [sda] 121110528 512-byte logical blocks: (62.0 GB/57.8 GiB)
sd 0:0:0:0: [sda] Write Protect is off
sd 0:0:0:0: [sda] Mode Sense: 45 00 00 00
sd 0:0:0:0: [sda] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
 sda: sda1 sda2
sd 0:0:0:0: [sda] Attached SCSI removable disk

If you use this, please make a donation to him for his good work: