Ok good, that must be where the problem is. Test for continuity across switch 3 on S5 to see if the problem is with that component or if it is open somewhere else…i’m assuming only switch 3 is on currently. If the pins of switch 3 on S5 are not connected flip on switch 4 or 2 just to see if you can get signal through the LPF. You may have to re-adjust the low side trim pot.
EDIT…i just re-read your post and it seems like you tested across the switch already. If so, test from R15 to R21/22/23/24…it should be connected. Then measure the resistance across R22 to see if it is really 18K. Then test across the switch (switch 3 should be the only one connected, do it again for good measure). Then test to see if the other side of S5 (the opposite side from the resistors) is connected to pin 3 of U2. …just figuring out where in that signal path it’s not getting through. My guess is S5 because that’s the only component with moving parts.