This works on my machine:

$ string="$(iwconfig wlp60s0 | grep -I Signal)"
$ echo $string
Link Quality=68/70 Signal level=-42 dBm
$ echo $string | cut -d' ' -f4,5
level=-42 dBm

For your machine replace wlp60s0 with wlan0.
Note this is using Ubuntu 16.04 and Ubuntu 19.04 where there is a space between 42 and dBm. As such the cut command is instructed to print fields #4 and #5. In your question there is no space so I’m not sure what version you are using.

You could also use:

$ echo $string | cut -d'=' -f3
-42 dBm

If you want to use ${string...} though the correct syntax is:

$ echo ${string##*=}
-38 dBm

$ echo "${string##*=}"
-38 dBm  

Either method will work to take the substring after the last =. The original method of 5 in your question I don’t understand how it can work.