It it is the wireless signal strength you mean, there are now commonly two frequency bands used.
the 2.4 GHz band has been used since the early days of wireless connections. The band is also used for many other devices, such as baby monitors and garage door openers. In many places the band is very crowded.
Because of this , another band, at 5 GHz, has been allocated. This allows higher speeds, but you need both ends to support it. A disadvantage is that the higher frequency signal has a shorter range, and does not get through walls as easily.
So it is possible that your computer has always supported 5 GHz, but the old router only had the 2.4 GHz signal. With a 5 GHz signal now usable, that's what the PC now goes for.
The signal strength may be lower, but the data rate may actually be HIGHER. You would need to actually measure the rate to make sure. There are numerous "Broadband Speed Test" sites which can do this.
If you are actually at the limits of the range, it may be possible to get your PC to prefer the lower frequency, or in some cases to get the router to stop broadcasting on the higher frequency. Some routers even let you have different names (SSIDs) for the high and low frequencies.
Another thing to check is the orientation of the router. The newer signal is more directional, so rotating the router might help.
