Could be caused by interference in the 2.4 GHz band in a dense populated area.

For example, I installed CPE510 and CPE210 to create a high speed link over 600 meters. While the CPE510 could reach maximum possible bandwidth of 90 Mbps, the throughput on CPE210 dropped to 3 Mbps due to limited AirTime (there were ~120 APs in the 2.4 GHz band in scope of the CPE210). So, I would use CPE210 only in sparse populated areas.

Regarding TX power: the CPE210 provides 11dBm, its antennas have a gain of 9dBi, thus total EIRP is 11dBm + 9dBi = 20dBm (equals 100mW EIRP). That's the maximum radio power allowed in the EU. 27dBm can be reached only in countries which allow more power.

There are two reasons why you shouldn't change the country settings of the CPE210 to exceed legal limits:

1. If authorities catch you, there will be heavy penalties (including seizure of all your WiFi equipment).
2. Increasing TX power over the legal limits will increase interferences in dense populated areas. This also increases the change that a neighbor will inform authorities, which in turn will locate the station causing those interferences - see 1.