jdobrzanski wrote

Can I connect this EAP120 with EAP115 (it supports cluster mode)? Or cluster mode is possible but only with one set of the same type of EAP?

The latter: cluster mode is supported only with older firmware versions of EAP115. Latest firmware removes cluster mode even from EAP115 in favor of the EAP Controller. Beside this, cluster mode didn't improve the WiFi coverage, it just was simplifying management by cloning the settings of one EAP to all others. And this is what causes problems - all EAPs use the same channel, same SSID etc. That's definitely not helpful for improving coverage. To extend the coverage, just buy another EAP (e.g. EAP110), set it to the same SSID, but to another WiFi channel. No need for clustering.

jdobrzanski wrote

Now, can you specify case where I would need to set up a cluster?

For lazy people wanting to clone the settings of one EAP to several others. But cluster mode is gone - it was available to other EAP models first, then to EAP115 only, then removed by latest firmware even for EAP115 for no known official reasons. I think it caused too many problems cloning an EAP setup to all other EAPs (just a guess, I never did play around with cluster mode).

Also, in the example you suggested (same SSID, different WIFI channel), if a device switches from one EAP to another (while moving at home), will that be transparent or there will be a temporary connection drop?

Connections will always drop since the WiFi adapter must close one wireless link and re-establish the new one. But since TCP/IP uses error recovery and retransmissions, this might be a problem only for connections using UDP or multicast.