After longer investigation. I think in your case missing (+) in Mobile menu seems to be caused by small CSS mistake from our side, sorry for that. Please use below custom CSS lines:
.menu-toggler::after {
content: "\f055";
font-family: "Font Awesome 5 Free";
font-weight: bold;
}
.offcanvas-inner .sp-module ul > li span.menu-toggler {
display: block;
position: absolute;
top: 50%;
right: 0px;
transform: translateY(-50%);
cursor: pointer;
}
.offcanvas-menu .offcanvas-inner .sp-module ul > li.menu-parent.menu-parent-open > a > .menu-toggler::after,
.offcanvas-menu .offcanvas-inner .sp-module ul > li.menu-parent.menu-parent-open > .menu-separator > .menu-toggler::after {
content: "\f056";
}