what do you use for Q268x capacitor to provide enough energy during transmission? I use CAP-XX 0.12 F 4.5 V but I’m looking for something smaller/cheaper. A few months ago I watched Fatrack PCB and there was only a SMD tantalum D, probably low esr - I don’t remember the value of capacity but I see in online stores that 470uF; 10V; ESR:40mΩ might be worth to try.

Second question - do you use any supervisory device to monitor the voltage (connected to the reset of Q268x) in the power supply-powered devices?

So long as your power supply meets the minimum specified currents and ripple voltages as stated in the product technical spec, you shouldn’t have to worry about expensive caps. You may also find it cheaper to fit 3 or 4 smaller caps (100u) instead of 1 big cap. If your power supply can supply the 2A pulse currents with no delays, you can even get away with as little as 100u, however 300u is probably better.They don’t have to be expensive tantalum caps, if you can get higher values ceramics, they should also work fine.

We don’t use any supervisory circuits, if your supply is correctly designed and you have the required ESD protection, the supply shouldn’t exceed the recommended operating voltages of the modem. However, it can’t hurt to have a supervisor!