This was a while ago, but is it possible your dhcp server is not giving out the same IP? My heru integration has been rock solid for a year, but I've made sure the DHCP server uses a static IP for it (and all the other modbus tcp units).
You can run the "boost mode" that sets the fan speed to high for however long it's configured to, but to change the heat exchanger behavior you have to change the temperature setting. If you want override cooling, you could just change that to whatever its lowest setting is.
I have a setup with passive cooling (frikyla) using a heat exchanger on the supply air that circulates the brine from the ground source borehole. To trick the FTX unit into ramping up the fans I have connected a fake humidity sensor to the unit. This "humidity sensor" is actually an esp32 running esphome with a 0-10V output circuit on it. The FTX unit is configured with the "RH boost" that ramps up the fans if the humidity goes above say 60%. The esp32 reports a "humidity" that's actually 60+10*(room temp - setpoint), so if it's 0.5C warmer in the house, it tricks the FTX into thinking the humidity is 65%. This causes it to gradually ramp up the fans to max, and when the temperature goes below the setpoint, it ramps them down again. This whole thing happens in concert with an adjustment valve that regulates the flow of brine through the heat exchanger using a PID controller.
This system works quite well (unlike the "night cooling" functionality which just does not give enough cooling when needed, in my experience) although you need a bit of electronics knowledge to do it. I think there are other options, like some Shelly devices that can output 0-10V.
One thing worth checking is whether you have activated both modbus and the custom/cloud integration. I my case I had both, it was because I first tested the modbus but then decided to go for the integration instead. This seems to have caused issues that needed me to reboot the device.
I only have the modbus activiated but it seems to work better now. I have changed the update frequency for the device and it seemed to help.