Arris and Ubee MTA not working after changing from Hybrid Flow to Basic Flow

Basic 1 or Basic 2?

What DHCP options are you sending?

for Arris at least and Basic.1 you need
DHCP options:
domain-name
host-name
provision-server
krb-realm-name
dns-server

and the reverse DNS for host-name.domain-name needs to resolve to IP assigned to the MTA, which can be done with dynamic DNS updates from dhcpd.

// DHCPD.conf
// turn on DDNS updates
ddns-update-style interim;

// Setup a key
key dhcpupdate {
algorithm HMAC-MD5;
secret "your key here";
}
// setup a zone to update and use key to update, obviously will need a DNS server to update as well but is out of scope for this post
zone mta.domains.net. {
primary ns1.domains.net;
key dhcpupdate;
}

# Map option 122 values for booting packetcable Devices
option space docsis-mta;
option docsis-mta.dhcp-server-1 code 1 = ip-address;
option docsis-mta.dhcp-server-2 code 2 = ip-address;
option docsis-mta.provision-server code 3 = { integer 8, string };
option docsis-mta.as-req-as-rep-1 code 4 = { integer 32, integer 32, integer 32 };
option docsis-mta.as-req-as-rep-2 code 5 = { integer 32, integer 32, integer 32 };
option docsis-mta.krb-realm-name code 6 = string;
option docsis-mta.tgs-util code 7 = integer 8;
option docsis-mta.timer code 8 = integer 8;
option docsis-mta.ticket-ctrl-mask code 9 = integer 16;
option docsis-mta-pkt code 122 = encapsulate docsis-mta;

// set domain name
option domain-name "domains.net";
// Sets hostname to mta + mac address
option host-name = concat ("mta",suffix (concat ("0", binary-to-ascii (16, 8, "", substring (hardware, 1, 1))),2), "", suffix (concat ("0", binary-to-ascii (16, 8, "", substring (hardware, 2, 1))),2), "", suffix (concat ("0", binary-to-ascii (16, 8, "", substring (hardware, 3, 1))),2), "", suffix (concat ("0", binary-to-ascii (16, 8, "", substring (hardware, 4, 1))),2), "", suffix (concat ("0", binary-to-ascii (16, 8, "", substring (hardware, 5, 1))),2), "", suffix (concat ("0", binary-to-ascii (16, 8, "", substring (hardware, 6, 1))),2));
// needs to be provided but does not need to resolve, can use google.com if you want, but needs to be null terminated i.e. \000 and each part of the domain needs length before the value e.g \003 then mta
option docsis-mta.provision-server 0 "\003mta\007domains\003net\000";
option docsis-mta.krb-realm-name "\005BASIC\0011\000";
// DNS servers to do the reverse DNS lookup
option domain-name-servers 192.168.173.14;