What do you see in the gateway traffic page on TTN, or the gateway’s own logs?
If you see raw messages there which aren’t being decoded by TTN, it’s a LoRaWan level problem such as mismatched node address or secrets or frame counters.
But if you’re not seeing any raw traffic time correlated to your transmissions, it’s a node, gateway or radio settings problem.
Which one? While they sell some actual LoRaWan gateways, they are also a bit notorious for selling people only slightly cheaper “gateways” that are actually nodes only pretending to be gateways and unable to support proper LoRaWan. To actually support LoRaWan, a gateway needs to be based on the SX1301, SX1302 or SX1308 multi-channel/multi-SF concentrator chip.