I have always been a fan of Moxa for industrial ethernet switches and protocol gateways

Edit: Looks like they do not have a Modbus to BACnet gateway, but they do have a ModbusTCP to BACnet/IP if that is the protocol you are looking for.

https://www.moxa.com/en/products/industrial-edge-connectivity/protocol-gateways/modbus-tcp-gateways/mgate-5217-series/mgate-5217i-600-t

Here is their chart for all the protocols they have products for:

https://www.moxa.com/Moxa/media/Resources/DownloadFile/mgate-protocol-gateway-selection-table.pdf

I have used both RTA as well as SMC gateways to connect AB CompactLogix to BacNet MS/TP network. Are wanting your plc to the the master (initiator) on the BacNet network or a slave (responder)?

The RTA gateway is configured via a built in webserver. Depending on how you want to interact with the BacNet network (master or slave) you would choose one of 2 RTA gateways.

https://www.rtautomation.com/product/460msbm/

https://www.rtautomation.com/product/460msbms/

The Fieldserver (SMC product that gets labeled under many different brands) uses a CSV file that you load to it via its web browser. The fieldserver can be either a slave or master and depends on the configuration of the file that you load.

https://us.msasafety.com/IIoT-Gateways/System-Integrators/EZ-Gateway-Modbus-to-BACnet/p/modbus-bacnet

I have used more RTA gateways, but I think I actually like the Fieldserver better. I used a Prosoft branded unit. I liked being able to configure the csv file offline and then upload it as apposed to having to make all changes online with the RTA.

Here is a prosoft version:

https://www.prosoft-technology.com/Products/Gateways/Building-Automation-Systems/Universal-QuickServer-Gateway