Ex7700 connection to router is not stable

The EX7700 is a tri-band device, with two 5 GHz radios, each using 80 MHz bandwidth, on a 2x2 MIMO each. One of the 5 GHz radios will be bound to the same channels like your primary WiFi router, the second one must be operated within a reasonable channel spacing. With the Channels 36..48 already in use (the major lower channels allowing 80 MHz bandwidth), there is not much free space left in most regulatory areas, as DFS and/or TPC is mandatory for most applications in the channel range from 100 ... about  Ch. 144.  Above, only few regulatory regions allow typically a singe set of 80 MHz wide channels, in many regulatory areas, these channels are exclusive for max 25 mW EIRP SRD applications. A certain minimum channel spacing is required when operating multiple radios in the similar bands.

Beyond of the EX7700 extender, keep in mind that also the 6 GHz WiFi 6E mode, access points are allowed on pure low-power mode mode, permitted to operate at 24 dBm EIRP (6 dB -below- 5 GHz DFS power levels), and WiFi 6E clients at 18 dBm EIRP (6 dB -below- that of the AP). Many Wi-Fi 5 clients today already operate within these power levels, so WiFi 6E range will be affected by the slightly higher operating frequencies, and the 6 dB power difference (below DFS). You could expect Wi-Fi 6E 6 Gb (in low-power mode) to be around 42% of Wi-Fi 5 DFS channel range - this is perfect on open spaces, but limiting massively where wireless and Mesh systems ae not set-up on free, open space.