Can't add a new network interface on UTM 9

Question

We set up our UTM 9 on a VMWare VM with 3 network interfaces; when we tried to add a new one, all interfaces (existing and the new one) got disconnected. 
 We checked our configuration, reboot our UTM, then the whole VM, nothing works. 
 Last word we heard form support is that it is a "non supported functionnality" (how nicely phrased), as though adding capacity was unusual. 
 We couldn't get clear instructions on an effective workaround : do we need to re-install every thing, is there an effective and safe way to do it ?

Christophe Le Jeune · Accepted Answer

As said before, we tried multiple actions with Sophos support, none of which solved our problem. 
 The only change that worked was to make a full re-installation with the previous 9.4 version. I can't tell what changed with this 9.4 that solved the problem, but the fact is that it worked. 
 Cheers.