E-bike battery needs custom charge voltage, but how?
First you need to get the voltage up high enough, and then you need a power supply with a CC-CV mode. That is enough to supply the BMS, which will actually do the charge management.

For simplicity, you can just buy 33.6V 2A lithium chargers.  However you can also buy step-up modules that claim CC-CV mode - set up correctly, in principle they should work but I would want to test that behaviour thoroughly (for example, with dummy loads).  In particular, I don't know whether cheap BMSes can protect against all failure modes.

Given the fire risks of getting it wrong, I would tread extremely carefully here.