From Alge:
"Operating ranges and interferences
The ALGE WTN operates in 4.2 GHz frequency band, the same as WLAN. This is also used by
other services. The operating range as well as the operation may be disturbed by devices working at
the same or neighboring frequencies."
My guess is that 4.2 Ghz is a typo and they mean 2.4 Ghz. So the reason for the wireless signal degradation over time could be interference from wifi access points or cell phones searching for wifi.
The WTN is a mesh network, so you could purchase an additional WTN box and use it as a relay. e.g. Place it between your photocells and timing.
Otherwise your plan of removing the building as a source of interference is a good one.