By no means an expert with stove flues but it’s got to be simple enough. As a metal worker I would either say look for a standard fitting to connect two lengths of that diameter tube together from a stove manufacturer (probable push fit with a high temp bonding compound or riveted and taped), or weld it back together.
Welding is probably best done with mig if it is thin wall steel or a little inverter set if it is cast iron with the right welding sticks (expensive sticks but worth it) if cast heat both ends with a propane torch first - helps prevent cracking from thermal shock.
Of course you could just replace the whole length which might be quicker, simpler and less cost.