Local nearfield noise, dirty power source, high temperature?Metallic enclosures placed in the sun can get very hot during summer.
Current antenna, what type? Monopole with magnet foot, internal PCB antenna or external antenna, omnidirectional?What type of communication?
GPRS, nb-IoT...Any special frequency bands used at problematic sites?
Faulty own equipment or faulty antenna feeding (cable, connectors) is excluded?
Weak signal permanently or from time to time? is RSSI, received signal strength, logged or noted? Corresponding values for other types of communication can be RSRQ or snr.
Any strong transmitter placed close to same location?
Local celltower out of free channels? Can happen if a lot of people is at same location, all using the phone.
To improve situation in a effective way must the problem be defined or else will you only spend time and money to often little use..
A directive antenna such as a Yagi-Uda pointing at celltower will increase RSSI and suppress noise from other directions compared to an omnidirectional antenna.
An Yagi-Uda antenna do also suppress unwanted frequencies and by then reduce interfering noise as it is more narrowband then a simpler monopole/dipole.
Sometimes is a directional antenna unwanted as any direction is unknown and then is focus more on an omnidirectional antenna with high efficiency.
Is it possible to temporary place a cellphone hidden in your control-box at same problematic location and let it log RSSI for same frequency and operator?