How can I have the right ctrl key behave as the left ctrl key?

I would like to have the left and right ctrl keys to behave the same.
I already have the left key working as it should, in all the shortcuts, etc., but the right one seems to be completely ineffective.
I have looked in the keyboard layout options, but with no results.

I am on Linux Mint 20.2, with Mate desktop environment.

–addendum–

The result of xev is as follows:

KeyPress event, serial 35, synthetic NO, window 0x5200001,
root 0x7ac, subw 0x5200002, time 416256, (49,43), root:(2600,505),
state 0x10, keycode 105 (keysym 0xff20, Multi_key), same_screen YES,
XLookupString gives 0 bytes: 
XmbLookupString gives 0 bytes: 
XFilterEvent returns: True

KeyRelease event, serial 38, synthetic NO, window 0x5200001,
root 0x7ac, subw 0x5200002, time 416464, (49,43), root:(2600,505),
state 0x10, keycode 105 (keysym 0xff20, Multi_key), same_screen YES,
XLookupString gives 0 bytes: 
XFilterEvent returns: False

The output of xmodmap is as follows:

xmodmap:  up to 4 keys per modifier, (keycodes in parentheses):

shift       Shift_L (0x32),  Shift_R (0x3e)
lock        Caps_Lock (0x42)
control     Control_L (0x25)
mod1        Alt_L (0x40),  Meta_L (0xcd)
mod2        Num_Lock (0x4d)
mod3      
mod4        Super_L (0x85),  Super_R (0x86),  Super_L (0xce),  Hyper_L (0xcf)
mod5        ISO_Level3_Shift (0x5c),  Mode_switch (0xcb)