What do the specifications of this RF power meter mean?
Yes, what you need is an RF attenuator, you need to bring that +45 dBm down less than +20 dBm so that is at least 25 dB attenuation.

Usually RF attenuators come in 40, 20, 10, 5, 3 etc dB attenuation. You can put two in series and the attenuation adds up.

Here are some 30 dB attenuators with N connectors:



Here's an adjustable one:



Note that +45 dBm (in 50 ohms I assume) is around 30 Watt which is a lot !

You might need an attenuator looking like this for that:



Do study the datasheet of the attenuator before buying as you need it to have the proper connector, power rating, characteristic impedance (usually 50 ohms) and of course frequency range.