I used OpenMPTCPRouter to aggregate 3 LTE connections (via routers connected to directional antenna, with SIM cards from different operators) when I was living in a house in the woods before the war has started I had to leave Russia. Worked like a charm, giving me up to 180 Mbps or so. May not be that good for aggregating different types of links together, but for using multiple cellular connections it's nearly a perfect solution. BTW 5G 3GPP specs include MPTCP support, IIRC for aggregating connections going via different gNodeBs (base stations)