I'd guess it's one of two possible issues:

1. Ports blocked by the network provider. Some networks will block outbound connections on all ports and then explicitly allow some of the common ports (e.g. 80, 443). Some will also allow slightly less common ports for corporate VPN connections (e.g. 500 and 4500 for L2TP). Based on the documentation for Back to Home, it looks like it uses WireGuard underneath. For MikroTik, that uses a default port of 13231 (source). If that port is blocked, outbound by a network provider, the tunnel would fail.
2. Its always DNS. Based on the documentation Back to Home relies on a Dymanic DNS (DDNS) service. Some networks may specifically block DNS queries for well known DDNS services as a security measure. DDNS services were really popular with malware creators for a while and so DDNS services became a casualty of security configurations.

A while ago I decided that BTH only works well for 1 device (using wireguard at least) at a time.

I messed around and made different profiles (not sure of the actual name) for each device and that solved my problems.

Worth a shot.