"Can I revert to stock ISP firmware?"

Yes, but it is difficult. The 39Link39 build writes a protection flag. You must use the serial console (UART) to erase the firmware2 partition manually. zyxel nr5103e firmware update 39link39 exclusive

Log in with the credentials found on the label at the bottom of your router. Navigate to the Maintenance Firmware Upgrade Updating the firmware on a Zyxel NR5103E Go

⚠️ Before you update

1. Back up your settings – Factory reset may be needed after update.
2. Check your current version – If you need band locking or bridge mode, search forums to see if this update breaks them.
3. Be sure you're on the right network – "39link exclusive" likely means a specific MVNO or regional ISP – flashing it on a generic NR5103E could brick the device.