On SIDE_A I have a backup LTE connection, without proper public ip, only dynamic ip and I server with Wireguard. 

SIDE_A = mobile LTE connection, without public IP, behind carrier grade NAT 

SIDE_A_SERVER = WIREGUARD (connecting to sideb.dyndns.org) 

SIDE_B = VDSL with public ip + ddns (sideb.dyndns.org) 

SIDE_B_SERVER = WIREGUARD (cannot connect to SIDE_A, because no public ip on SIDE_A) 

Best regards, 

Adrian