GL-A1300 / Slate Plus and AP1300 use the same chip. If you don’t need to travel with it, you can also consider GL-AX1800 / Flint.
First of all, which external (5G) modem do you want to use - this is not entirely universal and different manufacturers’ modems need to be adapted individually.
There doesn’t seem to be a mature 5G USB modem on the market at the moment, most are sold as modules. Do you want to convert the USB interface yourself?
So, all I can say is that the GL-MT3000 / Beryl AX and GL-AXT1800 / Slate AX have been adapted to the RM520N-GL 5G modem module.