@kibato9999 and
@nex86 ...Is your PADEMU settings = "Global" and Enabled = "On" ?
What model number is your slim PS2?
MX4SIO with BDM(1) or with BDM3 works for 99% of people here who have tried, so there must be something at your ends. Use GUiFormat and format your SD cards to FAT32 and 16K clusters.
Just a suggestion, try this...
First, install (only) OPL v1.1.0 on your FMCB memory card, then load up OPL v1.1.0 as you normally would for USB games...
(Once on the OPL page where it shows the list of your games)
(1) press the "Start" button on your controller to enter "Menu"
(2) then click on "Settings" and check you have these settings below
(3) "USB Device Start Mode = Auto"
(4) "Default Menu = USB Games"
(5) "Save Changes"
Now return to the list of games and press the "Triangle" button on your controller to enter "Options" and check you have these settings:-
(6) click "Configure PADEMU"
(7) "Settings Mode = Global"
(8) "Enable Pad Emulator = On"
(9) now go back and "Save Changes"
Once all that has been done...
For SCPH-10000 to SCPH-700XX ...install OPL 1627 (BDM) on your FMCB memory card.
For SCPH-75XXX (or higher) ...install OPL 1629 (BDM3) on your FMCB memory card.
You can download OPL 1627 (BDM) or OPL 1629 (BDM3) here at the bottom of Maximus32's post: https://www.psx-place.com/threads/m...-ps2-sio2-interface.29210/page-15#post-275928
Now with MX4SIO inserted, via your PS2, load OPL Beta 1629 (or 1627) to your OPL games list, and see if this worked for you.

TnA will kick my ass for these suggestions, but you can try them anyway, you have nothing to lose and it could work.