Are you sure you have SPort fw version on your RX, not FPort?
As I understand there is no any difference in SPort between X and R series on hardware level. Several month ago I just replaced X8R with R9Slim+ and had no issues.
To what pin did you connect the telemetry wire?
Last night I tried several different versions of R9 Slim firmware both the standard and the flex but no F-Port. I used the wiring recommended in several forums for the PixRacer to tie the PixRacer In and Out together to the RX S-Port and then also run the Ground from the PixRacer FrKsy port to the R9 RX. This wiring worked on the X4R RX but does not seem to work on the R9. I did read somewhere that I think they implemented a half duplex config by wiring just the out wire from the PixRacer to the S-Port wire on the RX. I haven’t tried this yet but plan to. The R9Slim also has an inverted S-Port on the RX pin. Not sure if this is something to try as well.