Media converter maybe? (Like those $100 or so fiber to ethernet converters, I say this as it’s usually a modem/router plugged into the ONT‘s ethernet port that does isp to cpe authentication, tunneling, etc. so the ONT is just converting fiber (from isps OLT) to ethernet for something more common to plug into)

Modem comes from MODulator/DEModulator. It's taking a signal of one sort (electrical) and modulating light to send info, and also demodulating incoming light to receive. It's a modem.