That's about right.....you never get the as advertised speeds. This is simply because of lag, whether it be on the hardware (modem/router or network adapter) or line latency or degradation. I'd say with your 90 mbs is on par., I get about 90 Mbps download and 75 Mbps upload on the Speedtest app on the 5 GHz band. The ISP advertises 100+ Mbps speed.
As for your question goes; Adding a 2nd router is not going to bode well. Unless you turn it into a repeater or turn off the wifi of your ISP modem/router. Other wise you're going to have two different wifi networks.
You can buy wifi repeaters to extend the signal of your ISP wifi.....but you'll need to do some research and shopping.