I put the fans to blow in. My fan got filter.

The reason is because these fans can only helps to create better air circulation. It's not like the fans in CPU or GPU where it directly attached to the chip's heatsink which sucking-out will give immediate effect. Whether blowing-in or sucking-out it only create air circulation, by blowing-in the outside air which have dust will be filtered before going in to the modem. The remaining vents on the modem will have air going out so it prevent dust from coming in. Whereas if the fans were sucking-out air, the remaining vents will have outside air and dust going in to the modem, long term will have dust accumulated inside the modem.