each time (or most of the time) when I restart or shutdown my Rb951Ui-2hnd I need netinstall to get it to work again and re-update it and work with it, why does that happens could it be the software itself, or maybe because I'm using a 9v adapter instead of the original router adapter (does that have a relation with this problem)?

If the 9 volt adapter does not have enough amp rating it could cause strange behavior. The device uses max 7 watts with no attachments which is .8 amps at 9 volts. It has 24 watt max total or 2.7 amps at 9 volts.

Device is shiped standard with a 24V power adapter. Go figure.

Providing the board's regulator really is happy with 9V (and I guess it is since that's the spec), then 9V is close enough to the minimum that you need to pay extra attention to the supply.

All PSU's will drop a little voltage when the current draw increases, even if just momentarily. When the routerboard boots, as with many things, it will draw a higher current than when just running quiescently. If the PSU is a cheap one, especially if it has very thin wires*, the voltage can drop substantially. Since 9V gives only 1V headroom, assuming the 9V is correct and it's not a bit below to start with, it could easily drop below minimum at peak demand.

*Don't rely on the apparent thickness either. I recently pulled apart a cheap Chinese PSU that came with something, just out of curiosity. It was supposedly 5V at 2A. The actual copper inside the thick-looking wires was hair thin, and soldered so poorly that simply wiggling it once broke it off the PCB. The output capacitor was tiny. There's no way that thing could have managed 2A without dropping voltage.

Are you saying that the 8V minimum specification already accounts for being given less than 8V? If so, I think that's a very unwise assumption to the point that it's plain wrong to assume. I'm not trying to be argumentative but specifications exist for a reason and if they say 8V minimum then we can't assume anything other than 8V minimum (no extra 'headroom' below that). If you have 'insider knowledge' that the regulator circuit only really needs, say, 7V, then please say but even then that's not what the specs say which is what the rest of us need to work from.

We must supply the 'headroom' above the minimum with our chosen PSU.

I'm coming from the electronics side of things. I frequently choose regulators/designs for a project to meet given specifications, and often that regulator will bring its own too, for instance a lower minimum input than is required by the initial specs; that might or might not be written into the final specs. That could well be the case with the unit in question but we don't know that and should not assume that. Certainly the regulator component's specifications on the datasheet do not include some unstated additional headroom below the specified minimum. If the regulator design says 8V minimum input, it's 8V minimum and not guaranteed or even likely to function properly below that.

That said, I agree that not realizing a higher current supply is needed for a lower input voltage with a SMPS is quite likely.

OK, if we get bat on topic: if OP would test if powering device with at least 12V, that would give conclusive answer to the question if marginal power supply caused the problems or not.