The main thing is that the voltage and polarity is correct. The rated amps is secondary. The rated value is more of a capacity than a continuous output. You want the rating to be at least as high as the original adapter and could be higher. The biggest problem of going too high in the amp rating is these power adapters often don't have good voltage regulation (often don't really have any regulation). For instance, on some of these power adapters, you may have one that says 12v, but it might show say 16v or 19v when there is no load on it. If you went crazy with the rating (say 12v and 10 amp), the voltage may be too high since the modem wouldn't be enough of a load for the power adapter to correctly load it.
The modem itself likely has some regulation as well as protection for overvoltage and possibly for reverse polarity, but it is hard to know for sure.
In short, the 12v, 1.25A adapter would be fine as long as it has the right plug and right polarity. I wouldn't use the 7.5v one. I suspect that the modem uses mostly 5v internally (the voltage is regulated from 12v to 5v), but there may be portions of it that needs say 12v. You are best off keeping the voltage correct.