Faulty mkr or network problem?

After 2 weeks working perfect, last monday my mkr 1310 stop connecting to TTN v3. In the TTN console, 'live data' tab, the only feedback is the 'Accept join-request'. I upgrade the firmware (now ARD-078 1.2.3), update all lib's and run several and diferent sketchs but nothing change ... allways the 'Accept join-request' because the "int connected = modem.joinOTAA(appEui, appKey);" is allways false. I do not have access to the gateway, but I already try in 2 diferent locations with 2 diferent gateways already used in the past.
I removed all the sensors from the hardware, the mkr is "clean" now.
Running out of ideas ... need help. I'm beeing blocked in the TTN or is an hardware problem?

Many thanks.